Just finished up a 3 week solo trip to India. This was my first time in visiting and I went to: Mumbai, Goa, Varanasi, Agra, Ranthambore National Park, Jaipur, Jaisalmer, New Delhi. Here are some tips I learned during my time there:

Getting Around

Taxis - download the app Ola. This app is like Uber only it gives you the options to order a tuk-tuk or motorcycle as well. It's also great for getting a base price which you can then use to negotiate with a tuk-tuk driver on the street. Uber is also available in some cities although a bit more expensive then Ola. If you don't have access to Ola for a base price just half what ever initial price a tuk-tuk driver quotes you and aim for around that price.

Buses - If you're looking for a bus to get between cities I'd recommend the app Redbus. I've never personally used a bus in India but I've come across travelers who suggested this app.

Trains - If you're looking to book a train between cities I'd recommend the official IRCTC app/site. You'll have to create an account and then you'll be able to book trains. The app is useful because you'll also get a mobile ticket. Trains usually get booked quickly so it's best to book in advance if possible. Also keep in mind that some trains do get delayed sometimes for hours. I personally had a train from Varanasi to Agra which was delayed by 9 hours. Use this site to check if you train is on schedule and to check at what time you should reach your stop. Make sure your ticket status is confirmed.

Flights - Keep in mind that you'll need to show your flight ticket to enter an airport in India. You'll also have to get any bags you'll be checking in scanned so go a bit earlier as there might be a line for this.

Accommodation

I'd recommend hostels or guesthouses for solo travelers as they are cheap and great for meeting people.

Sim Card/WiFi

I'd also recommend a sim card with data as they are very cheap in India. I picked up a sim card from Vodafone and they gave me 2GB of data per day for 1 month for only 550 rupees (about $9) including texting and calls. With a price this low it's a no brainer. It's saved me so many times especially when the WiFi was bad or I was on a train. You'll need to bring your passport and know the address where you're staying when you signup. Also you'll need a contact with an Indian number to activate the sim for you. My airbnb host was willing to do this for me but you should be able to ask your hostel/guesthouse staff. Here's a useful link in setting up your sim card access point. Public WiFi is hard to find and when you do you'll usually need an Indian phone number to access or it doesn't work.

Food/Water

We've all heard of the dreaded Delhi belly. I was lucky in that I had no major GI problems during my stay. I only drank bottled water and made sure to check the seal. I ate street food but only places that were recommended by my hostel or I found on Zomato. Highly recommend Zomato as there are a ton of reviews from locals and you can even order food and have it delivered!

Communication

Meeting the locals and travelers is what traveling is about. Whats-app is the most popular text app in India and among travelers. Download it. Use it.

Dating

If you're looking to meet some locals and go on dates I'd recommend Tinder. It has a good presence in large cities and is growing.

General

Avoid the scams and being ripped off. Most people that approach you will act like they want to be your friend but nothing is further from the truth. They will eventually ask you to check out their "friend's" shop or that they can get you a discount or if you can "help" them out. Some will try to be your tour guide. Some will grab your hand and try to give you a massage or place a bracelet around your wrist. Some will ask if you want a tuk-tuk. Don't be afraid to say no or ignore them. Ignoring people is hard but it's necessary for your sanity in India. Be prepared to haggle for things. Some items like food have a fixed price but items like clothes, taxi rides, etc do not. As a foreigner you will most likely be charged double-triple the real price so be prepared to haggle. Don't be afraid to walk away.

India is a beautiful, crazy country. It took some time and experience to adjust to it but eventually I did and I enjoyed my trip. Feel free to ask me any questions. Hope these tips help out any future travelers!