Some posters have made this a bit complicated, I fear. Here's what you do, and what I've been doing for the past umpteen years.
Keep a scrupulous record of your income and expenses for each gig. I have mine on a spreadsheet. Income is what you receive as an individual, your share of the gig money. Outgoings will include travel at .45p a mile, any accommodation/subsistence, instrument insurance and misc expenses such as leads, batteries etc. Also don't forget to claim for travel to rehearsals and associated costs. Then at the end of the year see how much 'profit' you've made. At the end of the tax year ask HMRC for a short tax return form. It's, er, quite short - four pages in fact. You'll need to put down all your PAYE stuff and your employer should be able to help you with that, P11D and P60 for instance. On page three you'll be asked to account for any misc income and capital gains. That's where you declare your band earnings.
Just as a PS, will you have to pay contribute towards band expenses, PA for instance. Here I suggest you record your gig money net, ie what you get after bunging in for the gear. If you bring shared equipment into your accounts it could get needlessly complicated and when/if you sell any gear you'd have to declare income on that. So best leave it out.
When you become a world beating band it'll be different of course but hope this helps for now. BTW I account for all my music income, cash and all. Most cash payments are accounted for by someone along the line. The only fireproof cash payment is that which comes out of the bride's father's pocket.