how to create a custom login for your site

i have been looking around for a tutorial or something that explains it step by step but cant find one anywhere.

i want it so i have a login form for my site and users can upload pictures once they have logged in and it can keep info about the user like how many comments they have made and stuff like that.

does anyone know of any tutorials or somewhere i can look to find how to do this, or somewhere to start.

would be greatly appreciated. also im using a theme that i have made myself.