getting a new cellphone?

ok. so i want a new cellphone. i have cellone/at&t. so is it easier to use my friends old phone (idk what service she has) and put a sim card into mine, put all my info on it, then put it in her phone and use hers? (is this even possible?) or can i buy a new phone from my company, and keep the same plan and such. if i do buy a new phone, from the company, and keep the same plan, do u still have to pay the origional price? (like $200?) or can you get the $30 deals (even tho it says with 2 year aggrement)?? thanks so much!!