Newsletters are HTML, yes, but as people have been explaining to you, email programs handle HTML differently. You can't use modern HTML techniques in email and expect them to work across the board. If you're willing to work with tables and limited CSS, you'll probably be OK. But don't be surprised if you run into issues.

In any case, once you have your newsletter designed, you can simply save it as HTML. Sending it depends on what email program you have. For example, if you have Outlook, you can open the file in there and choose Tools>Send Web Page By Email. You'll have to look up how to do it in whatever program you use.

And here's some information on CSS support in email programs: http://www.campaignmonitor.com/css/