Is it normal that i am afraid of dating my friend?

"So I really really need help. I want my friend back with out breaking his heart. "
"then we've made a promise that ill marry him when we gonna grow up. "

Not sure that's really possible. You wrongfully mislead him to think you're in love with him and he's going to marry you one day. Those are extremely powerful emotions so he's going to take it very hard when he finds out you don't love him back.

The sooner you tell him you're not in love with him the better, seriously.
Don't let him waste his life on thinking you're feeling the same way when you don't.
That's not fair.

"Also is it normal to be afraid of dating somebody?"

Of course it's normal, especially at your age.

You don't want to lose a friend over this and personally it sounds like you aren't attracted to him at all and now you don't know what to do.

"Next morning I was at school exactly in front of my locker he came up suddenly and whispered "I love you", normally I should be so happy about that instead, it ruined my day, even I like him too, the truth is I am afraid of getting out of the friend zone."

Right, you're miserable about it because it's too much pressure and too much expectations on you and you can lose a good friend over this. You don't feel the same way as he does and you need to make that known to him.

You're kind of discovering that guys and girls aren't really 'friends'.
They're potential romantic partners. The older you get the more apparent this will become.
At your age sure you have some plutonic friendships and maybe these guys don't really want to sleep with you but it's not very likely.

Tell him you love him as a friend and of course you were joking about getting married because it's so far away and you're both to young to even think seriously about it. If you want to date him but are afraid of getting out of the friend zone because it can ruin your friendship tell him that and maybe you need to take it very slowly with him.

Let him know his friendship is more important than just some other guy you might date so you need to take it very slowly.

It honestly sounds like to me you don't want to date him at all and you just want to be his buddy.
If so tell him that too.

"P.s sorry for my English I am from brazil"

Your English is fantastic thank you for learning! :)

HTH.

Why were you talking about marriage?

No 15 year old has punctuation that clean. Try again mom.