You do not need to pay tax, unless you earn above the taxable threshold (£12,500). If you do earn above this threshold, your tax obligations will be automatically deducted from your pay so you don’t need to worry about anything.

In regards to declaring tax, you only need to declare tax to HMRC if you have had a second job whilst working for Stint. You can find more info on this on the government website. If you do have a second job, you will automatically be put on a tax code that will deduct 20% of your pay - you can contact HMRC about this if you are cumulatively earning under the personal allowance.