It really depends on where you are in the subject. What level math are you taking? For algebra 1 or 2, Khan Academy and other video resources are helpful, but the thing that'll really build your understanding is going through rules and definitions and then trying to solve problems while proving them with those rules. My geometry class in high school was structured somewhat like that and it was great for my understanding.
I would look over what you're expected to know, then watch khan academy or some other video resource (see the sidebar) and take notes/learn definitions. Once you have those definitions you should do practice problems until you feel like you have a firm grasp on the concept, and then do the work that will actually affect your grade.
