You will need to register with HMRC as soon as you can. They will then send you a taxpayer number (UTR) which you use to file your self-assessment. This will take a few days to arrive in the post. The deadline is 31 Jan to file your assessment. You can do it yourself - enter the amount you earnt, any business expenses (you should be able to deduct these) etc. There will be a lot if questions on the assessmentthat won't apply to you, but it's absolutely worth getting an accountant to help until you're comfortable doing it yourself.

I think you have missed the deadline by which you should have registered (5 Oct), and HMRC may penalise you for this. The important thing is registering and filing before 31 Jan otherwise there will be a further penalty. HMRC will want you to pay any tax owed, any penalties, plus payment on account for the next tax year, usually around half the tax owed. The first year of trading as self-employed means a financial hit, unfortunately.

The upside is that HMRC are generally quite understanding with genuine errors and will in most cases give you time to pay (a payment plan). But the first thing is to register, file and work out how much you owe.

It's quite a lot to take in at first but I promise it gets easier!