You should definitely talk to your teacher about it, as well as check out Khan Academy online for detailed explanations and videos of the subject you're working on.
Tough question to answer in just a few words. For starters:
1) Read assigned portions of your textbook several times; outline what you are reading
2) Make a list of questions about material you don't understand; get help (here or from your teacher) with that material.
3) Read your textbook or other learning material BEFORE you attend class so that you'll have some familiarity with the vocabulary and subject matter. Going to class without having prepared in this way almost guarantees that you won't understand very much.
Plenty more. If you want further dialogue, ask some questions and I'll try to respond with something helpful.
