Unlikely to get pregnant in your very first try. But if you want to be safe you can get an older friend (over 18) to buy you Plan B, its over the counter medication to prevent your pregnancy. But you MUST use it within 72 hours of possible conception I believe. However, if you cannot get your hands on Plan B, you cannot take a pregnancy test for about 4 weeks, so you'll have to wait it out.
You are very young to have sex, but im not going to condemn you for it like some people are. Your mother would be a great source of counsel, seeing as she knows more about sex then you do. If you simply cannot talk to her, you should find someone you trust you can talk to about it.
The younger you are when you start having sex, the more likely you are to become pregnant before you turn 18. Even if you are careful...