How to get 4G on my iPhone 5?

I've just upgraded my phone with T-Mobile and got the new iPhone 5. I only seem to be able to receive 3G.
Is there something else I need to do to set it up?

It depends on your area!

If you are in the UK, you have to add the EE 4G service. You mist be in the EE coverage area for it to work. Https://explore.ee.co.uk/coverage-checker

T-Mobile doesn't offer 4G for the iPhone yet. Until they launch their LTE, you're stuck with 3G. Be glad you can get that. Most people who have iPhones only get 2G/edge with the iPhone.

As I understand it BOTH TMOBILE & ORANGE only offer 3G

This is the "strange" part you only get 4G when you are on EE, so the IP5 will accept the EE sim card

How you go about getting the EE Excessively Expensive tariff, or even if you can transfer is something you need to take up with TMO.