What should I say? What should I do? 

• Food If you are invited for a meal, and you have special dietary restrictions, it is considered polite to inform your host in advance of the event. In restaurants, it is polite to ask for Kosher, vegetarian or other options.
• Punctuality / Promptness Being on time is important in the U.S. It's considered impolite to be late. If the event is a reception, you may attend any time during the specified hours. If you enter an event late, it's polite to say, "I'm sorry to be late" or request, "Please excuse me for being late."
• Expressing Thanks / Gratitude When expressing thanks to a host for an event or a meal, a short note is sufficient in the same manner you were invited: in writing, in person, by email or printed note. The level of formality makes a difference. Most formal: "I really appreciate ..."; formal: "Thank you for ..."; informal: "thanks for ..."
• Dining Out In the U.S. dining out or "eating out" can be a form of entertainment. If an American friend asks you to "go to dinner," you can assume that you will split the costs or "go Dutch". If the invitation is to "take you to dinner," you can expect the other person to pay for your dinner or "treat you."
  • Who Pays? It's very typical between co-workers and small groups of friends to pay separately in a sit-down restaurant. This removes the burden for one person to pay for all. However, it's very important to inform your server before ordering! "We'd like separate checks please, if that's okay." An easier way to handle this situation is to simply divide the final bill evenly among everyone, regardless of individual consumption, if all agree. This can be done at the end, unlike requesting separate bills beforehand. Don't forget to include an appropriate tip if you pay individually. Note: Most restaurants automatically add the gratuity to parties of 6-8 or more. This should be printed on the bill. If so, there is no need to leave an additional tip!
  • Too Much Food! Typical sit-down restaurants in the US, especially popular "chains" like TGIF, Applebees, Outback and many others serve very large portions. For this reason, it's common to take the extra food with you. Most restaurants are happy to provide a small container for this purpose. 
• Tipping In the U.S., most food-service (at table-service restaurants only) and hospitality workers (bell hops, taxi drivers, hotel housekeepers) earn most of their income from tips, not a salary. What they are paid by their employer is usually only enough to cover taxes and other charges to deductions from their income. Therefore, they expect a tip of 10% - 20% based on the total bill. Failure to tip properly is an insult; failure to tip at all is a huge insult.

• Bathing and Hygiene Most Americans take a quick shower and use a deodorant or antiperspirant daily. Therefore, Americans are not used to natural body odors, and find them offensive in public. Some international visitors may want to adjust their routine for bathing and hygiene to avoid an awkward situation during the time they are here. A related practice is to brush teeth 2-3 times per day to avoid bad breath around others you may be talking to a close range. 
• Social Equality Unfortunately, the American dream of equality for all has not yet been achieved completely. However, Americans strive for individual respect for all, regardless of occupation, handicap, sex, race, religion, or sexual orientation. This practice does not mean we all agree; it means we respect individual differences. All individuals you meet here deserve the same level of respect, whether they are professors and doctors or janitors and housekeepers.
• Women Women in the U.S. have an active part in community life, including full time careers outside the home. In many cases, both parents take care of small children and share housework. In the workplace, women expect the same respect as men, and there are federal laws in place to prosecute individuals who discriminate based on gender in the workplace.

• Fashion The way you dress and behave leads others around you to make certain assumptions about you that may not be true. Wearing more revealing clothing may cause someone to think you are more open to intimacy than others. Also, wearing certain brand name fashions may lead people to think you are unapproachable or reserved. That said, U.S. culture emphasizes individuality, uniqueness and encourages people to develop their own personal style.
• Shopping Most retailers in the US will allow you to return an item for refund or exchange within a certain time period, if you have the original packaging, price tags and the store receipt. There are certain items you cannot return, like swim-suits, under-garments and partially used cosmetics etc ... Just ask the store clerk if you're unsure. Before buying any electronics, it's always important to ask, "what is the return policy?"
• Friendships One of the best ways to meet Americans is outside class in the cafeteria, before and after class, through student organizations and in residence halls. Friendships form around shared interests, work, study and opinions. By asking questions about life in the United States, you will understand the culture better and meet new people. Some will become friends. International students report it's often up to them to initiate a conversation with someone new. Americans usually appear warm and open. Friendship patterns are generally casual due to the mobility and quick pace of American life. An American may have a several acquaintances who are "friendly," but only a few "close friends." Among college students, flexibility is valued in friendships. Try not to feel offended if someone is ignorant of your country -- its level of development, its politics, even its location! Remember that many young Americans haven't had the opportunity to travel outside their own state, much less the U.S. Young students today appear very naive about world events and politics. Remember that both of you are learning about each other's culture, which requires time, patience, dialogue, and exposure. It will be tempting to spend time with people in your own nationality group. They share many of your own problems, concerns and interests, and can provide needed support. However, some international students spend so much time together that their English hardly improves. They form their own group in a foreign environment and only do things together that they'd enjoy back home. Many international students find, once they leave the United States, that they wish they'd spent more time learning from and getting to American friends.

• Dating "To date" or "to go on a date" with someone implies that you would like to know the person better, often leading to a romantic relationship. It does not mean that you intend to marry the other person. Going to social events together "on a date" indicates you enjoy the other person; it doesn't imply a serious commitment. For heterosexual relationships, it's traditionally been the man who would invite a woman on a date (cinema, dinner, parties ...) although it is not uncommon for women to take the initiative these days. Whoever asks for the "date" should specify the time and place. Recognizing that few students have a great deal of money, many couples "go Dutch" on dates, where the cost is split or shared. Physical contact between people can be hard to interpret. It depends on the individuals and the amount of affection developed for each other. Touching someone may appear to be an invitation to greater intimacy; sometimes it is not. If a female ever says "no" or "stop" or "that's not okay" or implies this message in any way, the man should never insist or force anything beyond that point. Doing so could be grounds for legal claims involving sexual harassment or rape if there is any sign of forced physical contact or injury. That said, a woman can also be accused of sexual harassment or other charges towards a man, but this is rare. Even though the U.S. is progressing in the acceptance of same-gender relationships, there are many who find it offensive or inappropriate, especially in public. Same-sex affection such as simply holding hands in public should be at your own discretion; be sensitive of when and where you do so. Attention to this point could also be an issue of personal safety. Unfortunately, many people who see any form of same-gender affection might become angered to the point they could physically threaten or act-out against you. If you have any questions about dating or any other American customs, do not hesitate to ask a fellow student or the ISS. Social customs in the United States are vague and confusing. It is not unusual to be confused at first.
• Taboos In general, Americans are uncomfortable discussing certain topics in social situations where there are mixed guests. To avoid an awkward situation, it's best not to openly ask about an individual's: politics, marital/relationship status, age, sexuality, income, religion(s). Once you have established trust and comfort with an individual, these taboos are much more relaxed.
• Personal Space / Touchiness In general, Americans expect direct eye contact in conversations and greater personal space between two people standing during a conversation. Traditionally, a firm hand-shake with the right hand well-clenched between two men is normal. Women may embrace each other cheek to cheek in public. Hugging between men or women is a sign of a deeper, more emotionally connected friendship during a greeting or parting. In U.S., unlike in many countries, men and women do not kiss on the cheeks or hold hands in public.