Since you are very young and this will be your first time using cocaine, start out by doing a match head sized bump and see where that gets you. Re-dose every 20-30 minutes or so, so you can accurately and safely gauge the purity of your coke. Most street cocaine is not very good anymore and for all you know it could be cut with some random amphetamines or some other dodgey stimulant-research chemical. If you know where you're getting it from though chances are it'll be okay; but the safest thing you can do is do small small bumps and go from there.
You can always take more, but can't undo it once you've taken it. And taking too much coke, especially if you aren't experienced, can be a very unpleasant experience.
Not trying to scare you or anything, so don't take it that way. Just making sure you realize there are potential dangers, and the best way to go about avoiding them.