Firstly, have you been doing revision up until this point? Don't feel guilty if you haven't - most haven't trust me.
You should be spending as many hours outside of lesson as inside, so if you have 15 hours of lessons, you should roughly be doing 15 hours of revision a week (includes hw), it's a lot, I know, but break it up into small pieces. For each half an hour you revise always take a 5minute break, text friends, get some tea/drink, scroll through facebook, whatever suits you. But never spend more than 4 of these sessions solid i.e after 2 hours + 20mins worth of break, stop and give yourself a much much longer break, or just take the rest of the day off
. But, remember this includes hw, so it can seem a lot, but it's advisable especially if you're starting late and want to increase those grades.
Try doing past paper questions initially and seeing if you can do them, the ones you can do make sure you fully understand why (e.g guessing doesn't really count), the ones you got wrong/are stuck on either try learn it yourself or better see your teacher and bug them with the question, they're there to be bugged! Now from this you should see an overview of what you need to revise/go over and then spend maybe the next week or 2 weeks going through those topics you got wrong, and then after that 2 week period try another paper - hopefully your grades should have gone up, and continue this routine until you're smashing every paper! I would recommend doing 10 hours of revision in the week (split how you like) and then as much as you can on the weekends, but remember no longer than 2 hours of pure studying and give yourself at least a few days off every once in a while!. If you're having trouble with procrastination, treat it at it's source give whatever is stopping you from revising to your parents/friends and tell them to keep it from you.
Hopefully that helps, I know it is a lot of work, but that is at best, starting with 5 hours in the week of just pure revision s fine too(just make sure you're not spending that whole 15 hours or whatever it is on hw) just make sure that homework comes first, and of course you need some social life. Good luck with your exams! Bug your teachers!