Depends on what country you live in, if you are in the UK and have already registered as self-employed you just declare it in your self-assessment, typically you will be able to offset any tax due against your personal tax allowance but if you make a game that goes viral then remember to keep a little back for the tax man as you will breach your allowance.
When filling in your self-assessment you will also need to add any PAYE income, Tax Credits and other income, don't forget to claim back expenses, you DO NOT need an accountant to take care of HMRC, it's a myth, an expensive myth, just keep track of income/out goings and ask employers for a P60, tot up the figures and fill in the online assessment when due, don't be late, the fine goes up every year!
You will also have to pay class 2 NI, if you have a PAYE job then part of your NI is already paid but self-employed NI cost a little more, again offset this against your tax allowance at the end of the year, my NI is around £650 a year but I don't pay it until the assessment is done and it's deducted from any due tax rebate, on average I get around £1800 a year back in rebate and expenses.
20 years of self-assessment and I never had a problem.
