First of all keep in mind this is NOT a game so don't complain that it doesn't have 32156 parameters or 2352 possible actions. This is just an interactive virtual pet to have some quick fun.
Choose between the three initial pets and take care of them to make them evolve in one of their 3 possible adult forms making a total of 12 pets available. Pets will evolve around 6 mins before you buy them but can grow up faster and into cooler evolutions if you keep their stats high.
Passwords to get adult versions w/o having to evolve are:
crocoquail, zombiecrab, wirx, kat, morgoh, skunky, grayshork, murtle, jellybag.