As others said, You're means "you are" so you need the article "a" - You're a good friend meaning someone is a very good friend to you.

But we can say "i am to do smth (i have to)" so why i cant say "i am to be someone..." if "Be" is a verb as well i am to be doctor (i must be)

"You're a good friend*" would make sense. "You're to be a good friend" does not make much sense.

"You're a good friend*" would make sense. "You're to be a good friend" does not make much sense.

Both are not correct English. Without any examples it is impossible to correct them.