Go on 'womens-health . co. uk /depo' That might answer some of your questions.

And I wouldn't listen to the others- it's not abortion, and it's your choise what you do.
Although it prevents you getting pregnant, the major only problem with it that I can think of is that it doesn't prevent you from getting any STDs, so to be propper safe you should use a condom also if you choose to go forwards with the injection.

Can I ask, how old you are? Although it's kinda irrelevant. And do you have a boyfriend?
xXx