It's up to you to pay it yourself or have friends invited pay. But you have to know this point :
If inviting some ones, it may mean you or your parents will pay.
If the group of your friends suggested it, it might mean they'll pay.
If invited, it might mean bringing some gift for your birthday. ^^
Conclusions :
If you invited, you have to be ready to pay for all of them. But your purse is not thick, of course it will be as you are just 15, and if you don't want to have your parents pay, what you can do is following :
a. reduce the number of people invited to fit to your purse if you have no mind to have them pay.
b. If the invited are very close friends, then tip them to pay a bit for what they'll eat. ^^
c. Otherwise, throw the party at your home economically.
You invite your friends for your birthday, But they feel burdensome for your birthday gift. Then in addition, have them pay for what they'll eat is not a good manner. I'm afraid those invited might blame you and about invitation behind you.
Suppose you are invited to your friend's birthday. You have to bring a gift and you have to pay for what you eat??? Will you go there? If the inviter is your very close friend, you'll willingly. But if not, you will speak ill of it, won't you?...^^