"How to host it?"
You need a "host" obviously, which is a machine that will be always on in order to give access to the server itself; there you will have to put your server files, database and so on. This can be done even from a home computer, yet you need to have a good upload/download speed, and keep it on 24/7.
"How to add user registration on my website?"
You must have a wamp or xamp server installed, and throw the .php files into the "www" folder, and then configure the files with your credentials so that they can connect to your database.