1. Medications: Always consult your doctor regarding immunizations that should be taken like Hepatitis A and Polio. Additional vaccinations that your doctor may recommend are Measles, mumps, rubella (MMR), Tetanus, Rabies and Hepatitis B. Also carry medications for malaria, diarrhea, anti-allergy, Typhoid etc. Keep a copy of those prescriptions with you in case the local authorities like the Customs question you. Things like keeping an extra pair of glasses can be helpful. Carrying general medications for headache, nausea, sunscreen, few band-aids and mosquito repellent is also suggested. Although you get everything in India but its just better to have them beforehand.
2. Tourist Visa: Citizens of most countries including United States require a tourist visa on their passport. You should consult an Indian Embassy close to your location. Following Indian embassy sites are helpful: www.indianembassy.org, www.indiacgny.org, www.cgihouston.org, www.cgisf.org, chicago.indianconsulate.com.
3. Make copies of all your important travel documents (like ID, passport and visa) and leave it with a friend or family in case you lose the originals. Fill in the emergency information page of your passport. If you know the hotel phone number you will stay, leave that also with them. Country code (phone) of India is "91".
4. Keep the phone number of all the four U.S. embassies in India i.e. Delhi (North India), Chennai (South), Mumbai (West) and Kolkata (East), visit these websites newdelhi.usembassy.gov/, chennai.usconsulate.gov, mumbai.usconsulate.gov/, kolkata.usconsulate.gov/ .
5. Register yourself in the USA State Department’s travel registration website https://travelregistration.state.gov/ so that they can use to assist you in case of an emergency. Also read public Announcements or Travel Warnings on the travel.state.gov website.
6. Do not wear expensive jewellery and money, do not put all your cash in one location. Get some US Dollars converted to Indian Currency (Rupees) before you go. Carrying some travelers' cheques is also a good idea. If you need money in India only exchange it with authorized agents or banks. Inform your credit card company and Bank that you will be traveling to India on which dates so that your card is not blocked for suspicious activity.
7. Always drink bottled water and make sure that its sealed. Always eat well cooked food. In India, do buy fruits cookies and crackers with you, Local brands like Parle and international brands like Britannia and Nestle are safe to eat.
8. If traveling in the monsoon, carry a raincoat and umbrella. Carry flashlight and an extra set of lock and keys. If you need directions prefer shopkeepers over pedestrians. Its a good idea to re-verify with someone else also. Winters (December to February) can be chilly, especially in North India. Carry sweaters and jackets.
9. Be safe: Follow local laws and keep patience, Do not leave your luggage unattended in public places, beware of pick pockets and do not accept packages from strangers. Women travelers should be extra safe.
10. Tipping: Tips are not common in India as in USA. If you want to tip try to tip not more than 10-20 Indian rupees.
11. Beggars: You will find them quite often. Although some are genuine but at times people are forced into it. You you want to donate, its better to donate to a nonprofit organizations.
12. You can expect to bargain/negotiate price of the item in most of the shops (expect for government-run). You can even start with half price. Leave if you are not satisfied.
13. Try to learn some local words. In north India Hindi is the primary language, Southern India has a separate language for each State. Lot of people in India speak English, specially in popular tourist spots.
14. Not a lot of people follow the traffic rules, so prefer not to drive yourself. India has a lot of budget airlines, consider flying when traveling long distance. Trains are also a good option. Try to travel in 1st or 2nd class compartment instead of general class.
15. Cell (Mobile) phone coverage is quite good in India, and cheap too. Do consider getting one if you are staying for more than 1 month. Most plans have free incoming calls and cheap outgoing calls. If you want to buy a Sim card and unlocked international phone consider prior to your visit check a good site telestial.com.