Can you help me concentrate more when doing tests?

Hello. In my country high school basically makes your future. The best one is the American College, because you get an American diploma. The test looks alot like the SSAT. you have 1 part reading comprehension, 3 parts math and 1 part essay. I'm very good at math, have some awards at competions this year, but make very stupid mistakes(i go to this course were they give us tests). Can you tell me how i can concentrate some more? And give me some tips on being better on the reading comprehension, please. Also i seem to be wasting my time lately. I surf the web and look at stuff i desire. Waste hours reading reviews, watching videos about them, and then give up on buying it and the cycle continues. I also think im addicted to the comp. But i'm happy im realizing this now, because i have around 10-11 months to prepare and get my affairs strait. Can you help me?!