Authorization in http-functions

Hi. I am creatin a forum website. And I develop the mobile app of the website. I developed all a register API and login API. When I receive a request from mobile app with sessionToken, I want know who is the user and what is authorities of that user? When I try to delete a post wix doesnt see if the user owner or not of the post. Please help me? I can give more detailes about the stuation if this is not enough. Thank you for your help.