You need to create a .ascx file. In this file you are going to lay out the html portion of the control just as you would in any .aspx file and then code it the same way you would your .aspx file.
Now when you want to use it you need to register it. This is also really simple. You need to write out a tag kind of like this one:
< % Register TagPrefix="my" TagName="control" Src=".myControl.ascx" %>
You can name ‘TagPrefix’ and ‘TagName’ just about anything you want, and ‘Src’ is the path to the control. And now to actually use it: