The W3C has a good introduction.
In short:
HTML is a text markup language. Text means any characters, not just ones in ASCII.
- Save your text using a character encoding that includes the characters you want (UTF-8 is a good bet). This will probably require configuring your editor in a way that is specific to the particular editor you are using. (Obviously it also requires that you have a way to input the characters you want)
- Make sure your server sends the correct character encoding in the headers (how you do this depends on the server software you us)
- If the document you serve over HTTP specifies its encoding internally, then make sure that is correct too
- If anything happens to the document between you saving it and it being served up (e.g. being put in a database, being munged by a server side script, etc) then make sure that the encoding isn't mucked about with on the way.
You can also represent any unicode character with ASCII