If you've started a business in the last financial year (start of april to start of april) then you'll be due to make a tax return before the following January (if filling online — it's by the end of october this year if filling by post). Filling online has got to be easier, as the calculations are done for you, but that doesn't mean that it's simple.
If this is your first year's filling online, then be aware that there is a delay of about a week between making an account on the HRMC website, and being able to fill in your tax return.
First you'll have to wait a few days to receive an activation code. This is sent by post after you make your account at www.hmrc.gov.uk, and an activation code is sent for each online service that you sign up to use. I can only assume that it's to check that the address you gave is at least recognised by Royal Mail, possibly as some form of physical CAPTCHA. There will then be a further delay of up to a day, from when you activate the service with the code, before it becomes available for you to use.
Also, if your accounts for the previous financial year consist of a pack of receipts in a cardboard box grouped as “General”, “Misc” and “Other” (Black Books S01 E01) then you'll need to calculate your total turnover, the value of your business expenses and the cost of any capital purchases.
Business expenses is split into 9 categories:
Rent, rates, power and insurance
Repair and renewal costs
Fees for professional services
Other allowable expenses
You will also need to know the value of your purchases of capital equipment. These are dealt with separately from business expenses because they can only count against your taxable profits to a certain value (capital allowances).
If you make a loss, it is possible to "bring it forward" to another year to offset profits made in a subsequent year and thereby reduce the amount of tax you will pay. So if you have made a loss in a previous year, be sure to include it in this year's tax return, or make a note of any loss this year to include in next year's tax return.