Welcome to the board, zwarrior99...
Let's see if I have this straight . . . You have injectable B12 in your possession, but you don't know how to use it, right down to wondering whether you should inject it in a vein (no, you should not).
Sit back for a minute and take your medicine - this is in your best interest and will do you a lot of good.
First of all, you're 16 years old. Why would you want to shoot B12 in the first place? What leads you to believe you need it, let alone that it will do any good? Why are you possessing it before learning anything about it, including how to use it? And why haven't you learned how to do a basic Internet search, not to mention a search on any anabolic board (including this one) that uses vBulletin software?
Okay, bro, end of lecture. Now click on the Search link at the top of this screen. Enter cyanocobalimin as your search term and TNT (c'est moi) as the author. You'll find several threads in which I discuss injectable B12 in detail, including how much to use, what type of needle to use, and where to inject.
Then, after you've done your homework, post any additional questions you might have and I'll be happy to give you answers without the lecture.
And befiore you hear it from anyone else, you're 16, dude. Don't be so anxious to shoot anything. Read up on steroids here on A.R. and you'll find out why almost everyone recommends that you do not use steroids at your age. You've got a lot of potential - don't blow it by not doing your research and listening to the guys and gals who have been at this game a lot longer than you.
Again, welcome to the board.