You can improve by being diligent about taking good notes in class and doing your homework assignments. But a lot of it has to do with natural aptitudes. For some it comes easily and for others it's a struggle.
Get a tutor. That helps...A LOT! Like garo1 said, make your basics strong. Math (especially Algebra) builds upon those foundational skills. And of course...the more you practice, the better you get at anything.