Part of the problem is, what is this letter going to be for? Are you emailing someone, or is this going to be a normal "mail" letter?
If it's email, go to your email, and there'll be a button labeled "New Message." Click that and you'll get your "clean page" for your message. Enter the email address of the person you wish to send the message to, a subject, and then your message. When you are finished, click the "Send" button
If you want to send a formal printed letter, you need to run your word processor. This could be MS Word, or even Word Pad in a pinch. Or you can download one for free called Libre Office.
Once you run the program it will display a new document and you can start typing.
It is always a good idea to save your work often. Go to File -> Save, to save your file. The first time you save it will ask you for a filename. Type in something that will be meaningful to you (e.g. "letter to mom")
When you finished you can print the letter.