Easy-peasy!

1 - Log in.
2 - Click on "post something! [1]"
3 _ Choose "forum" or "story" (that's the front page)
4 - Type away!

By default every members posting box is in "rich text" that is, with the HTML helper "tinyMCE" added. If you click on the buttons at the bottom of the box you'll get help bolding, linking and even adding images to your post - works a lot like word which I'm sure most people are familiar with.

If you dislike having the default html-helper "on" you can turn it off on your "my account" page. Look for the "tinymce" setting (orange link)