Indent 4 spaces!
You can also click the little code button above the editor, or press CTRL+K with the code selected to do the same.
Which is linked from
- the little [?] icon above the editor
- the full reference » link in the sidebar when asking or editing
EDIT: I realized I didn't fully answer your question. For tiny code snippets you can use backtick to escape things to code
Anyway, we disable intra-word emphasis in Markdown, so what you're seeing in the preview is not in fact true in the final post.