A very, very late reply, and it's too simple, but :
Hmm...Just try to keep all your worries/problems/bad thoughts out of your mind while meditating. And even if they find their way through, you have to believe that it'll all be alright, and that you shouldn't fuss over whatever problems you're having. It's not like tomorrow won't come. The world won't end if you make a mistake, embarrass yourself or don't manage to solve a problem or two right away.. Be optimistic, and try to think of everything good tat's happened to you..Be it some good memories, people you love, hopes you have for the future,..etc. It'd be good if you concentrated on a certain goal you'd like to achieve, and think positively about it.. Try not to get too unrealistic or it'll be bad....Or you can just cancel the thinking, not think about anything at all and just relax! That works for some people too! :3