Start with counting as the above comment says. Spend some time on counting as questions are tricky.Next go for logic,its easy and you can solve mostly with intuition.
Sets,relations and functions are studies during our +2 and it is easy as well.
Lattices and group theory is the area you might want to spend some time and graph theory you can do it anytime after counting.
Refer Rosen book and its solutions are available online.
Good luck,happy reading