How to write a letter of resignation??

Question

I want to leave my current job. I know I have to write a formal letter and also give two weeks notice. 
How do I go about doing this? Where do I even start on writing a letter saying I'm leaving? What do I include? Aghhh I've never had to do this before! 
The worst part is my partner has been offered a job 160 miles south of us so he's moving down next week so they're going to think it's because of that (I was going to wait and follow him down when I went on maternity leave in September) but my health is declining a little due to pregnancy (I have hyperemesis so been throwing up to the point of fainting at work! And constantly feel sick! But I also have hypermobility so standing on my feet for 8-18 hours a day 6 days a week is just getting too much! I've already talked to management and had my hours reduced but it's not really helped

Answer

It only needs to be a few sentences:
Please accept this letter as a formal resignation of my position as (job title) on (date.) I have appreciated working for (company name) but due to personal health concerns I'm unable to continue employment at this time. Over the next two weeks I'll be happy to assist with any necessary transitions due to my impending departure.

Answer

Thank them for the time you spent there, then follow with, unfortunately, my last day will be (x) due to my need to focus on my health at this sensitive time. And sign it respectfully. It doesn’t need to be elaborate ☺️

Answer

I hate how jobs say you HAVE to give a letter of resignation like no boo I’m leaving i don’t have to do anything it’s really just being nice

Answer

To whom this may concern,
     I would like to thank you for the opportunity to work with this company; it has been my pleasure. Unfortunately, my circumstances have changed and this is my notice of 
 resignation on (this date).  I am sorry for any inconvenience this may pose to you or my position. Thank you for your time. 
                              Sincerely,
                               Your name

Answer

Thank them for your time and the opportunities you had there. It's not really a big deal of why they think you're leaving, you just are and that's all they need to know about it  Google can give you a structured layout and you just include the wording. make sure you include the last date that you will be there. "Please find this letter as my resignation from "position". My last day will be "date".

Answer

You need to include the medical reasons for leaving your job.  I would also think long and hard about leaving for temporary reasons.  See if you could qualify for short term medical leave before just quitting.