I heard that, only internet explorer support iframes. Maybe, Netscape 6.2 support, too, but not sure. Anyway, let's start with the basic iframe.
<iframe width="number" height="number" src="URL of your site" name="name" frameborder="number"></iframe> frameborder="number"></iframe>

width="number"
Put the width of the iframes. You can use only "px" or "%". 
i.e. width="50" (it's already px you don't need to put 50px) or width="50%"

height="number"
Put the height of the iframes. You can use only "px" or "%". 
i.e. height="50" (it's already px you don't need to put 50px) or height="50%"

src="URL of your site" 
This is where you want to open the first page in iframe. Put the link of it.
i.e. src="main_menu.html or src="http://www.yoursite.com/main_menu.html

name="name"
Put the name of the iframe. You can open your others link of your site here, too. Just make sure that the target of those links are same as the name of iframe.
i.e. name="main" or name="top"

frameborder="number"
Here you put the borer of iframe. It can be any number.

scrolling="no" 
You need this only if you don't want to let your visitors scrolling.


I guess, this tutorial will help you to understand iframe more easy. If not, I am sorry. Actually, this is my first tutorial, so, sorry again, if I confused you.