I'll try to help you with the best of my knowledge.
First off:
Yes, you can indeed use a phone such as the LG Chocolate with a cell phone service provider such as T-Mobile. However, the phone must be
unlocked in order for you to be able to use it.
Now:
An unlocked phone is a phone that will work with any SIM Card that is inserted into it.
A locked phone is generally any phone you get from the official store itself, and will only work with that specific company's SIM card.
So, if you buy a phone at the T-Mobile store itself, most likely that it will be locked and will only work with T-Mobile SIM cards. SIM Cards from other companies such as Cingular will not be compatible with your phone.
And just for basics, a SIM Card is generally that tiny little card that is inserted behind the phone's battery. It basically just stores all your phone's data.
Now onto the LG Chocolate:
The LG Chocolate is originally a
locked CDMA phone provided by Verizon. What this means is that the phone is built to run on Verizon's network.
The problem with this is that because Verizon uses CDMA technology, it does not use SIM cards. I believe another company that runs on CDMA as well is Sprint.
If you use T-Mobile or Cingular, you are required to use your SIM card in order for your phone to work.
Now, to be able to use the LG Chocolate on T-Mobile's network, you'll need the GSM version of the phone, which means you'll need the unlocked version.
You can get the unlocked version from many different sites, some such as
Ebay or
LaLa-West.
Assuming you already have a T-Mobile phone with their service, once you have bought the Unlocked LG Chocolate, just reach under your original phone's battery for your SIM Card, insert it into your new LG Chocolate, and it should work work fine :3