Generally speaking, the best time to visit India is from October to mid-April. Summer season is from March to June, Monsoon from July to September, October is again a warm month and winter lasts from November to February.
WHEN NOT TO GO.
April - July: Avoid North and South Indian plains, as the heat can be unbearable
May - September: Steer clear of coastal areas
December-February & July -September: Hill Stations
October - May: Ladakh
GETTING A VISA.
Yes you'll need a visa. Valid for six months from date of issue (not from your entry into the country). Multi and single entry often cost the same. Many people do a double entry so they can pop into Nepal for a respite from the hassles, annoyances and heat of India before returning to finish up the north.
If you're American, do yourself a favor and send away for your Visa with plenty of time to spare. India will not give you a Visa on arrival and the embassy Visa process can be a harrowing experience, especially around the holidays.
Restricted area permits are required for the following states in India: Sikkim (15 days, get in Siliguri), Andaman Islands (30 days - issued at the airport if flying in, if coming by boat you'll need to get in advance). Permits are no longer needed for Assam, Tripura or Meghalaya, but you still need one for Manipur, the Arunachal Pradesh, Mizoram and Nagaland.
Most locals speak English or can quickly produce a brother or an uncle who can get your message across.
INTERNET & COMMUNICATION.
Loads of internet access, but often slow. Phone calls home a breeze to make from numerous call shops and international rates very reasonable. Most guesthouse will also accept incoming calls if requested.
You can find loads of ATMs and credit cards can be used for large items such as plane tickets. In tourist areas virtually any hard currency cash changes (you name it CHF, AUD, CAD and so on), but Euros, GBP or US$ will be easiest overall. Also worth mentioning is the importance of smaller denomination rupee notes because change can be hard to come by, so break larger notes when you can and hang on to the small 10 & 50 notes.
If you’re in a bind, head into a travel agency and ask for a cash advance from your credit card. Most will do it for a small percentage.