This applies to the UK and I’m not an accountant so you should verify any of this before acting on it…
I understand from a friend who has a charity role with a (very) small income that HMRC are no longer interested in miscellaneous income that’s below £1000/year (in total). I guess that at low incomes the tax that can be levied (probably less than £200 on average) outweighs the cost of processing a self-assessment return. So if you’re paid through PAYE and a little extra income would mean you have to complete a self-assessment it’s too expensive to do that so it’s not necessary.
If your forum turnover will be more than £1000 then you can follow what is called a simple bookeeping approach. Basically just record all incomings and all outgoings related to your ‘forum business’. It doesn’t matter how you receive the money, e.g. as cash, cheque, bank transfer into your own account, payments into another ‘business account’, Paypal, etc. Just record all the transactions along with details of what they are/who paid you, etc. You probably don’t want to set up a business bank account because those are likely to add costs, so using a personal account is fine. Just be prepared that you might need to provide evidence that you’ve recorded all income so you may have to provide marked up personal bank statements showing forum related income and costs.
You can only claim reasonable expenses, so you’d need to investigate what ‘allowable expenses’ are for running a forum. For example, it’s difficult to see how you could justify including transport expenses (company Rolls Royce) for running a remotely managed forum server, but the cost of running the server and paying for domain registration fees is likely to be allowable. Also be aware that paying for a server and using part for a public forum and part for a personal project, e.g. your own email and blog, means you can’t claim the full expenses for the server…just pick a reasonable split that you could justify.
If you’re running this on your own, then register as self-employed with HMRC. If you’re running with others you can register as a partnership but I don’t know what this means these days as it’s 15 years since I was in a business partnership with anyone. You’ll have to start doing a self-assessment return each year, if you’re not already doing one. Before you do the assessment, just do a simple balance sheet for the financial year - a spreadsheet is good enough. A few columns are needed - Date - Description - Income - Expense. Add up Income. Add up Expenses. Subtract Expenses from Income for Profit. You’re best leaving your own 'financial year as the same as the HMRC tax year, i.e. based around 5th April.
On the self-assessment form say you need the Self-employment pages.They’re simple enough…basic info about your self-employment, your turnover (Income) and your expenses, and I think they ask for Profit too. They will calculate how much tax you owe and if there are any National Insurance implications.
Source: I was self-employed for many years whilst running a forum (with something else tagged on the side). I did my own accounts every year and never had any queries from HMRC about how I was doing it. It saves on the cost of paying an accountant if you’re confident enough to do it yourself.
Bonus Tip: If your expenses exceed your income, then you have a negative profit (loss) which can be offset against any other income you have, e.g. PAYE from a day job which can save you paying a bit of tax. That was certainly true for me for a few years!
PS. If your forum is making ‘big bucks’ then the above probably won’t apply!