Call HMRC within 3 months of starting month. So that means if you start now you have until the end of April to tell them. Anything you have spent now can still be claimed as a business expense, but it's probably a good idea to do this as soon as possible, maybe the start of February when you've sorted out your bookkeeping.
Expenses (part of your outgoings) are anything you spend money on for the sake of your business. Products, insurance, rent, rubbish collection, advertising, leaflet printing, mobile phone, etc. Buy a large A4 book, and a folder. Every week have the left page for sales and the right page for outgoings. Whenever you get some money in then write it down on the sales page and date it, plus say if it's was cash, cheque, credit card. On the outgoings page whenever you spend money then write down what it was (VERY IMPORTANT, DON'T FORGET) and the date again. Then put receipts, invoices etc for that outgoing in the folder as a record. If you take money out of the business account for yourself then write it down as Drawings. When you get your bank statement just check to see all the sales have been banked and that you haven't been overcharged for any expenses.
If you are going to keep a cash float then also write down how much is in it every day in a separate little book. Keep that separate from the sales and outgoings book. Think of it as a second bank account. So say you take out £20 at the start of the month and put it into the float. At the end of the month your main bank account should show all the sales you have deposited, and the float will have any cash you haven't deposited PLUS that original £20. So you can keep as little or as much as you need in the float, you just need to record it all the time.
That will get you through the first few months until you are used to how it works. Then you can move onto a spreadsheet (or just stick to this way, and change to weekly sales instead of daily).
To do your tax return you get your A4 book, add up the sales you took for the tax year and write it down. Then for your expenses/outgoings do exactly the same. Then enter them in the self assessment declaration (they tell you where to enter the figures). Whatever is left is what will be taxed. So take care when you draw out money for yourself, you should leave enough in so that you don't go into overdraft or have to go without a wage for a while when the tax bill comes.
The tax year is the 6th of April to the 5th April the following year.
If you don't fancy doing it then record everything as above and give it to a bookkeeper or accountant to file your self assessment.