Make sure you get it in on time, otherwise you face hefty penalties. Here's how to get it sorted.
1. Set up an account
The paper deadline has been and gone (31 October) so you will have to fill out an online self-assessment tax return. To start, you will need to register with the HM Revenue & Customs (HMRC) website at online.hmrc.gov.uk if you haven't already done so.
You will need your unique taxpayer reference, which can be found on any paperwork you have from HMRC. You'll also need your national insurance number.
After you have registered you will see your user ID displayed on screen. Be sure to make a note of this. You will then receive an activation code in the post - this might take several days so make sure you start well ahead of the deadline.
2. Sort your paperwork
You'll need a few things to hand to fill out your tax return, including details of pension contributions, charity gifts, savings and all your sources of income, including investments and buy-to-let properties.
If you are employed and are only filling out a return to cover supplementary work you'll also need your P60 from your employer. It shows how much tax you've paid through your salary.
3. Don't miss the deadline
The deadline for completing your return and paying what you owe is midnight on 31 January 2013. If you owe less than £3,000 and want HMRC to collect it through your tax code then the deadline is 30 December 2012.
If you do miss the cut off you'll be charged £100 if it is only a day late, and £10 for every day after that up to a 90-day maximum of £900. If it is more than six months late a further £300 or 5% (whichever is higher) of the tax due is charged. And you'll have to pay all of these penalties even if you have no tax to pay but have to file a self-assessment form.