I can name three very important things you should always bring with you when you travel to India (or anywhere in the world, for that matter).
1) A durable and secure backpack. Your backpack will be your companion throughout the trip (if you are traveling solo). All of your most important items like wallets, passports, cash, cards, etc. will be inside your backpack. This is not to say that there are a lot of thieves in India but you need a durable pack to secure all your important belongings. It's also recommended that you use a pack lock to secure the inside compartments of your pack.
2) An updated guidebook. There are hundreds of guidebooks about India. You should get at least two. Make sure that the ones you avail are updated and accurate.
3) A map. Yes, you can use apps in your smartphone but it's always a great idea to get a traditional map.