Before you leave for India, it's important that you are vaccinated against any diseases that are preventable. Most American tourists need to be vaccinated against TYPHOID FEVER, HEPATITIS A, and have all of their other vaccinations up to date. If you plan on spending any time out in the wilderness, it's also suggested that you have a RABIES vaccine. Talk to your doctor about any others that you might need.
Schedule the vaccines *at least* 2 weeks before you go, but preferably earlier- your body needs time to build immunity.
One more thing you need to research is preventive malaria medication. Most places in India, with few exceptions (mostly areas in the north) have a "malaria problem", so travelers are urged to begin medication before their trip. The medication does not "prevent" malaria, but rather kill the malaria when it's still in its early stages.
Now, this topic might be a little disgusting, but it definitely needs to be addressed- make sure to pack an anti-diarrheal with you. No matter how careful you are about the water you drink, and the food you eat, you still are very likely to develop diarrhea during your stay in India. It's just the way it is.