if you are a beginner, your forward roll is supposed to suck, also your backward roll on at least one side, look at the fine print on your contract, i think it is item # 24
besides your instructor, other good sources (sometimes easiesr for nervous beginners to ask) are senior students--- I'd suggest one whose rolls you like, that you feel comfortable working with. and don't be surprised if you get different/conflicting advice if more than one person helps, we all do things a bit differently---if it gets too confusing, smile when you tell the others that Student X is helping you so you'd like to concentrate on what s/he told you to try.
I would try to do them before and after each class (still do, you can never roll too much), and in the beginning to keep a positive mind set, would end before i got tired (more likely to make mistakes) and after a roll i was fairly satisfied with, so my last memory of rolling would be good.
Good luck, Vera!