Howto (legally) do "nixers" whilst on PAYE

Hi, I am a full-time PAYE worker and am considering a new, separate, out of hours venture which could see me charging for a service. How do I go about this tax wise? Do I need to setup as a company? Not really sure where to start. Thanks.

You could register as a sole trader for income tax.
You would continue to get your tax credits etc for your PAYE job, and then pay the additional tax on your self employed income each year.
You would have to prepare accounts and submit a Form 11 tax return.