I don't quite know if this is the right place *ahem*

This is my first web development project and I am building an online community and want to implement user profiles.

My question is twofold.

Firsly, if I decide to implement my own, how do I keep track of who is who while they are on my site? They register and their details are passed into my database but how then do I know who they are when they try to visit their profile page or post a comment?

Secondly, exactly how do I handle the security issue? I've read a little about hashing/encryption but I'm treading new waters and not sure what to do. Any advice (even if its 'suck it in and use google/facebook sign in') is so appreciated.