user managing with mysql+sessions

hi! i'm trying to make a user-configurable site, where people must register/login if wants to change the "looknfeel". the user's data will be stored in a mysql table. ok, registration will check the data, and then it's stored in the table. and then the login. but what to do, if the login was correct (i checked that the username and the pwd was correct). how can i manage it with sessions. or is there another ways to store user data... thanks, kojot ps.: if it was discussed earlier, just tell me (maybe when)... i'm new for this...