preparing my tax return for first time - help!

Ok so im employed and work full time but run fitness classes 2 nights a week and altho i dont earn alot i want to make sure i do everything right to keep me covered so have registered as self employed and planning on submitting my tax return online. As my classes are paid in cash im finding it difficult on how i prove my income. I have a 2nd account set up where my rent money for the studio that i hire comes out, my electric, mobile direct debits come out, i keep receipts for anything i have to buy. I just have a cheap receipt book where i keep note of money received so i can keep track on how much ive earned and i bank the money every 2 weeks. Is this receipt book enough evidence and obviously my bank statement will show the cash being banked? I have bought an account book that i find very confusing. I was planning on just entering the amounts i received onto an excel spreadsheet with the dates for each month cus i maybe take in say £500per month but my rent, phone and electric comes to £200 a month, £300 profit and with 20% for my tax is £60. Im maybe thinking too much into it, its just after reading on hmrc website if you dont have correct information kept in your records you can get fined, i dont really do it for the money its more a hobby and dont want to end up getting fines unnecessarily, this part is really taking the fun out of it for me!