Hi Sourab,
Before choosing the affiliate program, you need to decide on a service or product that is in high demand, yet very few websites promote it online. This will ensure your marketing efforts do not go to waste, especially on PPC engines - the quickest way to make a buck - where top spots are going for $2-3 a click nowadays.
Clickbank is a good place to look for niche products. To start on your niche finding mission, have
Overture Term Suggestion Tool and Google ready. If you see something that catches your eye, see how many search results you come up with in Google and Overture for that product's keywords. Contrast these two numbers and go for products that have high demand (overture) / low supply (Google). You can also have a look at how many Advertisers are placing ads on Adwords for your keywords. If you can get on the first page without spending a lot of money, then you might have hit a winner.