If you need to write a letter to request some time off work for sick leave, it sounds as though this may be for a serious or ongoing condition, in which case I am assuming that your doctor is aware of it?
If you are seeing a doctor about the condition, they should be able to help. A GP can make the call as to whether you are "fit for work", and if not, can provide you with a certificate which says this, which you can give to your employer. In which case you would not even need to write a letter.
They will usually sign you off for a short period - perhaps a week a two - and then towards the end of that period you would see them again, so they can assess if you have recovered sufficiently to go back to work.
If you have not been to see the doctor, or do not wish to, you could just write a letter to your employer, but if you want sick leave, ie. You want to want to be paid for your time off, you will have to provide a doctors certificate this.