It's actually quite a nice process, you don't need to have done anything like a food hygiene cert etc before you speak to them; have a look at Birmingham City council's page here, it should be very helpful.
The way it worked for me was:
1: read over the initial guidance
2: give them a call, chat over what I need to do next
3: do the things they suggest and register with HMRC as a new business (you can do this online very painlessly and it automatically registers you for self assessment)
4: arrange for an inspection (and this wasn't like passing an exam; we talked through cleaning plans, managing pets whilst baking, how to manage the 'second, hand wash only, sink' rule without having to fork out on installing a silly extra sink in my kitchen........)
5: register officially as a food business (for you this will be done here)
6: make and sell your cakes!
Hope it all goes well; you're welcome to PM me if you ever need a hand with your HACCP; I found it to be the most frustrating part :-)
Best of luck!