Hi Tanaji. You're asking a few questions that suggest you are starting out learning about design and analysis for fMRI.

I would recommend that you find and read a copy of The Handbook of Functional MRI Data Analysis by Poldrack, Mumford, and Nichols. This book is a bit old now, but it gives a very clear overview and introduction to the area. Understanding these basics will make your life much easier.

Good luck!