4 Answers

If you are leaving because you hate working there, DON'T SAY THAT!  Leave on a positive note and don't burn any bridges. You never know when someone will make an inquiry about you at that company, whether you used them for a reference or not. Lots of people know lots of people, especially in the business world!

You could just write "I quit" or follow Vinny's good advice............

Ducky

...or "Take this job...." lol

Start nice how you appreciated the chance to work there

give your reason for departure

set a date for your departure

and a thank you.

Giving reason is optional, although your next job they will want to know why.

"This letter will serve as notice of my resignation, which will be effective as of (date). 

It is customary to give at least a two-week notice.  If your job is not tenured, your boss may choose to terminate your employment effective immediately.

I don't think a reason for leaving is necessary, but appreciation for the opportunity, the experience gained, or whatever you gleaned from the job helps close the book gently.

Hopefully, you're moving on to something way better, or experiencing a joyous event in your personal life which makes your job unnecessary.