HELP! How to change network/tower info on SGH t809?

My friend recently gave me this phone, its basically new. He ordered it off ebay along with a few other unlocked phones, but never used this one. The problem im having with it is that i hardly get any bars/service with it. I think thisn is because the phone was originally from Europe. I put an antenna booster ont he back of the phone under the battery and still no luck. Somebody told me that ic an change the network and tower settings but im not sure of how to do it. Does anyone know how i can fix this problem and change the towers to my area? If so please get back to me. Thank you.