Some things you should know:
Cows. There are cows everywhere. Big ones. They're in narrow alleys, blocking your way. There are herds of them crossing the highway, ignorant to all approaching traffic. They're eating huge piles of smelly trash off the sidewalk of a busy street. And they're treating the entire country like their toilet.
Shoewear. Don't wear flip flops in India. Just don't. See above.
Poverty. You will probably see some of the poorest, saddest people on Earth in India. There are children who are dirtier than stray cats roaming the streets, begging incessantly for something to eat. It is heartbreaking, impossible to avoid, and sometimes, it is so constant that it admittedly becomes annoying.
Dress. Indians dress formally. Women wear beautiful, embroidered saris while digging trenches on the side of the highway. You will never see an adult wearing shorts. Women are expected to cover themselves appropriately- do not expose too much of your shoulders, legs, or chest. Yes, saris expose the entire tummy. No, we Westerners don't understand why they can show their tummy and we can't show our shoulders. It's just the way it is.
Staring. Some Indian men stare at Western women. Cover up and ignore them. Or stare back, sometimes that can be fun!
Just say no. Indians can't. The closest you'll ever get is an "it is not possible." But in more cases than not, you will get a run around answer to a question instead of the person simply telling you "no." Try not to ask yes/no questions if you really want to get to the bottom of something.
PDA. Men and women should not touch each other in public. Touching, even just holding hands, is considered part of sexual relations and should only happen behind closed doors. However, men (and to a lesser extent women) can be affectionate with each other, and you'll often find two Indian men walking down the street holding hands; it's a sign of friendship.
Public Urination. If you're a man, go ahead and drop trou wherever you want. Seriously, don't hold back. The world is your toilet in India!
Adjectives. India likes prepending adjectives where they're not necessary or otherwise redundant. They say "Self Driving" when they really mean "Driving": as in driving yourself; maybe your driver is on vacation. They also say "Lane Driving" when we would just say "Driving": although there are lanes painted on most roads, they're more of a suggestion. They say "Love Marriage" when we would just say "Marriage": to distinguish from arranged marriages, which are still quite common. We even saw a university advertisement listing degree programs for "Engineer" and "Girl Engineer"!
One Photo, Please. We're still not sure why we appear picture-worthy to droves of adolescent Indian men, but such is the case. As a Westerner, you'll be asked more times than you may have patience for whether your photo can be taken. Then each young man in the group will have his friends take a photo of him with you on his cell phone. Grin and bear it!
Spitting and Hacking. The clearing of orifices that Westerns would only do in the bathroom-- spitting, hocking a loogie, and blowing snot rockets-- is a common public habit in India. You'll wake up and fall asleep to these lovely sounds.
Horn Please. Indians drive with one hand on the horn and the other on the gear shift. The driving in insane, and apparently no other vehicles/cows/people/rickshaws will know you are approaching unless you honk incessantly. Pack earplugs.
The Train Ticket Mystery. Buying a train ticket in India is like going to the DMV, but with rules that are 100 times harder to understand, and without the "take a number" system: your fellow patrons absolutely will not form a line, opting instead to mob the ticket window, everyone trying to push to the front and shove his form through the glass window to the ticket agent (these ticket agents must be the most patient bureaucrats in the world; they are friendly, efficient, and helpful in the midst of total chaos). But I digress.
The real issue is that there are boatloads of different ways to buy tickets: at least three online systems, separate ticket quotas for every stop along the train's route, plus a special one for tourists, something called TATKAL (a reserved block of tickets that only gets released the day before the train leaves) plus every travel agent seems to have access to a different ticket block. And if one of those options is sold out, it says nothing about availability through the others. In our five weeks in India it took us until buying our last set of tickets to have just the faintest grasp of this arcane system.
The World's Largest Democracy. Related to the Train Ticket Mystery, Indians pride themselves on having the world's largest democracy, and enjoy sharing this fact with those of us from the birthplace of democracy. For the tourist, what this means is that you're visiting the world's largest bureaucracy: be prepared for lines, forms, permits, "official" stamps, unnecessary rules, five-step processes that could have easily been accomplished in one step, and providing every last bit of your personal information in triplicate every time you check into a hotel.
Now you're slightly more ready for the most insane place you will probably ever (willingly) visit! Have fun, and remember: expect the unexpected.