Hello Chetan

The strengths of  carboxylic acids are known to vary based on the total power to withdraw electrons of the atoms that are bonded to the carboxyl group.

Ex:  CH group has a lesser electron-withdrawing power than H. So, CHCOOH is comparatively a weaker acid than HCOOH. Similarly, CHCHCOOH is a much weaker acid than CHCOOH and HCOOH.

Master Organic Chemistry http://www.masterorganicchemistry.com/ is a very good site for learning organic chemistry. You can also visit ATP on youtube and study some topics from there.