As people have already mentioned virtual racing would be a nice avenue to explore. More and more online racing leagues involving sims like rFactor have started broadcasting their races online, especially now that certain mods make it possible to display live timing and TV style graphics from those races on-screen. I have tried doing it a bit myself and wasn't very good, but some of the others in my racing league really sounded professional and were able to keep on talking.
There are a couple of websites around which racing leagues go to and ask if they can commentate on their races, so see if you can get in contact with them
E.g.
RLM TV
Quite a big one is
PSR TV
If you start there then (as others have said) you can start building a portfolio which will help when you start commentating on real events.
Good luck, hard work is what you will need more than anything.
EDIT: Oh wow I didn't realise that John Hindhaugh had started doing sim commentating, that's incredible that you got the chance to commentate alongside him Liam! Really good commentary!
