Hey, can anyone explain how one can search for idea & its feasibility from marketing perspective? Like if you are suppose to find out how you will search for right market ( & from where )?

First you need to find groups related to your market. See if there is a large market for your product by going to FB groups, reddit groups etc. If there are thousands there, and if there's engagement you have a good chance of making it. You can also ask google the market size and stuff. The key is market research.

interesting!! But what about if u don't have clarity or idea you want for your startup?

That's a tough place to be. If you don't know what you want to do yet find something that you like, and that also makes money. Pick a niche that you like that has a large market. Mixing what you like with a large market is a good combination and will put you in the right track.