to be honest if you are creating a login system you should really have a file for functions and config especially if you are using classes so just include session start in that or make a more complex one that will check if a session exists and if not starts it again. More complex systems will log info and session ids in the db.
Look at some login tutorials etc and you should get it pretty soon. Session variables work a lot like POST GET or standard variables as far as printing out comes but you need to understand how to set them, secure them and make them persistant.