Pick up math text books on algebra & trig (often called pre calc), calculus, linear algebra, statistics, and discrete mathematics. Ignore what ever subjects you don't need.

If your cheap and want instant feedback with unlimited practice problems then khan academy is a nice go to with a placer test that helps you figure out where to start.

Lastly youtube has a ton of content to help you better grasp a subject so be sure to abuse the shit out of that resource.

That being said as a CS student 3Blue1Brown's Essence of linear algebra series will be the best thing you ever watch.

https://www.youtube.com/playlist?list=PLZHQObOWTQDPD3MizzM2xVFitgF8hE_ab