Go and read a beginners tutorial on php (There has to be one around here somewhere), and mess around with some basic stuff on your server.... Or find someone to program stuff for you.... which will probably be hard, because there arent many people willing to work on a virtual pet site... =/ I helped out someone not that long ago, and then it made me look bad because they were kicked off the server for writing bad code... Not a good thing when it was me who wrote some of it (hopefully not the bit that was using a lot of queries.. =/)...
I have a permanent dislike for virtual pets... shudder
Oh, and for the login script, I dunno if its a good idea to use sessions because they end when you close your browser as far as I know (I'm not sure if you can make them last longer some how?)... I'd use cookies via set_cookie() but I dunno really if theyre better or worse....