The threshold is £1,000 per tax year, so April to April. If you earn over £1,000 in a tax year, register to fill it in online, as it's much easier than doing it on paper. If you do it online, the first bit is a few tick boxes about what your activities have been, e.g. are you employed, self employed, own shares, owe capital gains tax from something etc, and then they'll only show you the pages you actually need to fill in. It's a lot less daunting doing it online, as you won't be looking through a huge booklet and thinking should I fill in this page or that page etc.
As long as you have a "simple" situation (i.e. that you get paid once a month or per job, have a basic spreadsheet, and maybe have a couple of expenses, rather than having a big complicated thing with lots of stuff going on), then filling in a tax return is much easier than people realise. There's loads of help online if you google stuff, and HMRC have lots of helpful pages and videos.
The deadline will be 31st January 2023 (it's only 31st October if you do it on paper, I think). But always do it earlier than this, mainly because if you need to phone HMRC to ask them anything, if you phone during January you'll have to sit in long phone queues with everyone else who's left it until the last minute. Do it earlier and it's much easier to get help and advice more quickly.