I cant use app stor,i cant buy aplication free

Try restarting your iPhone. Have you signed in with your Apple ID? If not, see this article: Sign in with your Apple ID - Apple Support

If that doesn't work, then I'll need more information. What happens when you try to open the App Store?

If you would like help you have to provide information. What happens when you try to buy something ? If you are getting an error message what does it say ? If you are getting a message to contact iTunes Support have you done so ?