Another advocate for opening a new account with an existing bank that you do online banking with. It needs to be a current account with a debit card.
I started with £50 and have made £3k doing it very part time over the last year. Really must get on with more sign-ups!
You'll definitely need to use an exchange. Don't be afraid the exchange! The exchange is where you effectively bet that something isn't going to happen - so if i bet Ladbrokes that i thought Red Rum was going to win the national, I'd bet the exchange that i thought Red Rum was NOT going to win. Be careful, in football that's not the same as backing two opposite teams, because there are three possible outcomes on a football match not two... and no it's not as simple as using three bookies either 
Yes it's totally legal and you don't have to declare it as income on a tax return.
Team profit is the most beginner friendly website and set of guides in my opinion. The only thing about using them is that they're very Odds Monkey - orientated not Profit Accumulator, if you're got your heart set on PA. I've stuck with OM because I got a good price to sign up for another year, but now I'm more experienced I think Profit Accumulator is probably the better software.
Just take your time to understand it all before you start, and follow team profit guides precisely. The only time you can lose money is when you make a mistake. If you do exactly what the guides tell you, you won't.