How do I know if a PSP has wifi??

I don't have the PSP yet but i'm working on getting one but i was just wondering how to tell if a psp has wifi. does a psp come with wifi automatically or do i need to buy a reciever or something to go online. and also do i have to conect my psp to the wireless internet i have in my house or will it work automatically. basicaly i have no idea about the wifi for psp so if you could give me step by step directions on how i get internet onto my psp from the moment i get it that would be great.