Once the video file has been fully exported from Windows Movie Maker and the file is saved on a storage device, go to YouTube.com, and sign into your account. From the Home page, you can go to the gold Upload button in the upper right hand corner. When you drag the cursor over the Upload button you get the Upload Video File option, which you should click.
You will be taken to the next page which has a button labeled Upload Video, which you should click. This will take you to a storage device search window where you then go find your file and select it. From here you just properly title it, add the info necessary, and complete the upload process. It will then go on your YouTube video page where people can add comments and view freely.