Session variables

Hi, i've got an easy question and need someone with more experience opinion. I'm not sure when I should start a session for a new user. Do I start it at the HTML form when they register? OR do I wait until registration was accepted by the PHP page that processes the HTML form? Ditto when a user comes back, i'm assuming I have to check for a session variable using the PHP page that processes the HTML form sending a users login/password information. Well, I hope that made sense, i'm in the early stages of my project and would like a bit of guidance. Thanks in advance, Mike