Indian summers are hot. So, if you are planning to come in this period carry light clothes. Cotton works best. Shirts, T-shirts, Jeans, Cargoes etc. for the men. Skirts, pants, T-shirts etc. for women. Anything cool and comfortable. India is not a very conservative society so you can pretty much wear what you want as long as it's "decent." If you want to try Indian clothing, then, men can wear "kurta pyjama" and women can try "salwar-kameez" which are available in any shop.

Winters (October to February):

Indian winters can be quite cold, especially in North India. So, pack woollen clothes, jackets, pullovers etc. with you. Warm innerwear is only required if you are planning to travel to the mountains.

Monsoon (July to September):

This is the rainy season and some areas in South India including Mumbai and Goa can have heavy rains and there is a high level of humidity at that time. So if you are planning to travel in the monsoon carry your rain gear.

Last but not the least, while visiting places of worship (temples, gurudwaras or mausoleums), you should be fully clothed. In some of these places you have to cover your head. A handkerchief or scarf will do. Don't forget to remove your footwear before entering any religious place.