just go through the basics of every topic and start solving prev yr questions.I suggest you to start from these videos

LINEAR ALGEBRA

https://www.youtube.com/playlist?list=PLPiOSvowot1JLNguTkQUfXQJlLLML54yM

PROBABLITY

https://www.youtube.com/playlist?list=PLPiOSvowot1Lc6vs4tMzjTbUIVWVuuamb

SET THEORY

https://www.youtube.com/playlist?list=PLPiOSvowot1IRMhWIHuHQJTiLKiDpX1PJ