This is a good place to start:
Melevsreef.com - What is a Sump?
<Follow all the links, there are good links to discussion on sumps, DIY plans, etc.>
I would, however, suggest buying a HOB
overflow rather than building one. It is more challenging to do than what the websites would indicate. <Maybe I am more inept?> A 10 gal
could work as a sump, although depending on the size of your display tank, the working volume might be rather small once you factor in the safety room needed to prevent flooding .... so it might not be worth you while to set it up. Generally, the bigger the sump the better ....