Check your notice period
Your contract should say how much notice you need to give. If it doesn't, the legal minimum is one week once you've worked there for a month. You can agree a shorter notice period with your employer.
Put it in writing
Keep your resignation letter short and positive: the date, that you're resigning, your last working day, and a thank you. You don't have to give a reason.
Your final pay
You should be paid for the days you've worked and for any holiday you've built up but not taken. If you've taken more holiday than you've built up, your employer may only deduct it if your contract allows.
Garden leave and references
Your employer may ask you not to come in during your notice period, called garden leave, while still paying you. Employers don't usually have to give a reference, but if they do it must be fair and accurate.
Get advice before resigning if you feel pushed out
If you're thinking of resigning because of how you've been treated, get advice first from Acas, Citizens Advice or your union. Resigning can affect your options.
Common questions
Do I have to give a reason for resigning?
No. A short letter with your last working day is enough.
Can I leave without working my notice?
Only if your employer agrees. If you leave early without agreement, you could be in breach of contract.
Do I get paid for unused holiday?
Yes. You should be paid for holiday you've built up but not taken when you leave.
References: GOV.UK, Handing in your notice; Acas, Resigning from your job.