How to set up my Iphone 3gs?

I Just bought an iphone 3gs off of my buddy, perfect condition. Now what is the Best way to set it up to get the most use out of it? Should i use my old go phone sim card? Should i go through straight talk? Or can i get a contract through at&t? I don't know what is the best option to be able to unclock all of the potential of my new iphone.

I Just bought an iphone 3gs off of my buddy, perfect condition. Now what is the Best way to set it up to get the most use out of it?
--- if its already jailbroken, then just use your fingers and set it up the way you want it. DO NOT RESTORE. Restorals on itunes will wipe the jailbreak. If its not jailbroken, then its suggested to do a restoral to start out from scratch like you just got it at a store.

Should i use my old go phone sim card?
---go phone sim cards won't work in a Iphone, per ATT policy. Once the towers read a gophone sim # on a Iphone IMEI#, BAM… SIM is suspended until you call ATT and explain yourself. No joke!

Should i go through straight talk?
---if its jailbroken, yes

or can i get a contract through at&t?
--- contract services are better than prepaid. Cell Towers, on any carrier, will denote more resources and availablity to those who pay more $$$$$$$ per month. And since that fall to contract users first, you'll get better cell service than a prepaid carrier.

i don't know what is the best option to be able to unclock all of the potential of my new iphone.
--- Youtube video search… Google… Ask friends… Join an Iphone forum… Search appstore… Play with your phone! LEARN! Thats how we all do it.