So you stole this instead of buying it and getting a manual? I'd guess you should take a look at gamefaqs.com and find one of the faqs there that lay out the buttons. In short though I think you can use a stylus, but it's more confusing. B button does most stuff, and A backs out, and X is the menu, and L+R runs like hell.
I'd also suggest if you are a thief, start with FF3. FF4 on the DS is vastly harder than the previous releases as they introduced cheap attacks into the game where you psychically (or using a faq) have to know what only attack to use or you get beat to death in like 2-3 hits tops basically making all boss fights annoyingly narrowminded and choreographed. It's my favorite game of the series and hurts me having to tolerate that crap as I dont have the GBA, SNES, or PSX version anymore.