De-activating code in text

[J.M. Latham]

Sorry if this has come up before but ...

I need to be able to deactivate the code within text.

Example:  I am trying to write a guide in HTML re: how to use basic CSS.

When I type out the code to "comment out" text from a stylesheet in older
browswers, all my own work disappears.

I know there must be an HTML code that says "don't act on the following
code -- just here for amusement" but I can't find it ... well, not
quickly, at least.

Any help?

Joyce