You will either love it or hate it.
Don't drink the water.
I won't tell you not to give to beggars, but if you do, do it discreetly...otherwise a flock will descend upon you.
If you are buying shyt, never pay asking price... you are more than likely getting ripped off.
Definitely try and take a train ride, it's a great way to see the country, and the transition from village to city life before your eyes. Dont eat food thats offered to you though. If you feel brave enough, take a bus too, but im warning you not everybody is built for that.
I suggest checking out the south if you can, people are more educated and modern, and more likely to be friendly towards you. The south is more like "real" India culturally, no Muslim influence like you see in the north. It also has the best temples and scenery.