What happens if i put my Sim Card in a sidekick lx 2009?

I have At&T and my friend offered me a sidekick lx 2009 Unlocked
What happens if i put my sim card in it?
Will At&t add a data plan for me?
Will my unlimited text messaging still work on the sidekick?
Whats all the features i can get if i do this?

Im not really good at phones so can someone help me a little bit? :D

If the phone is unlocked it will accept the simcard. Only locked phones stick to a network. Recently I suffered with one such phone, but luckily got it unlocked from Simpleunlocking.com and removed the network lock. Whatever plan you had in your sim will work with this phone too.