T-Mobile's version of the Samsung Galaxy S5 just started getting the rollout of Android 5.0 Lollipop. The OTA started pushing out to users early this morning. The download is a hefty 939 megabytes so you will probably want to pull it down through WiFi.
Keep in mind that the update will be rolling out in phases so if it doesn't hit your phone right away, just be patient. Alternatively, you could head over to Settings > About Phone on your device to see if you are in the current wave of upgrades. Sound off when you get yours and let us know what you think!