Usually when you use an iPhone it is because you are on a monthly contract with a provider that sells them. These phones are generally locked so that they cannot be used on other networks. But is a way to use them on T-Mobile if you know how to unlock iPhone 3g for tmobile.
To unlock the phone call the network from which you purchased your iPhone and ask them for the unlock code for the device. They will ask for some information from you like your cell number and email address. This is so that they can send you the information you are requesting.
Once you receive the instructions follow them closely. Then press the power button down on the iPhone and slide the red tab right when cued. This will shut the phone down. Use a removal tool to open the SIM tray inside the iPhone. Then you can take the SIM card out of the iPhone and replace it with the SIM card from T-Mobile. Place it in the tray and press the tray into place so that it is flush with the outside of the casing. Turn the iPhone back on by pressing the power button. When it reboots it will be connected to T-Mobile.