If you want to keep your iPhone but use your android simply deactivate iMessage and FaceTime then take the Sim out and put it into your Android.
If you are getting rid of your iPhone you will need to deactivate find my iPhone iMessage and FaceTime and then wipe the phone. You will need to make sure that the device is paid up and unlocked before you sell the iPhone. However that's an entirely different matter.
Is the SIM card in your old phone is a different size than the nano SIM in the iPhone then you would perhaps need to get a new SIM card that will fit that Android and have it activated.
Once you activate a new sim the old SIM is deactivated and cannot be used again.
Manage your T-Mobile account