If you have Microsoft Office, use Visio, it's going to be the most complete product out there for creating network diagrams. If you don't have Office, you could try using one of the free services on the Internet.
Gliffy is great, I've used it before for quick and dirty diagrams - http://www.gliffy.com/
Draw.io is also another nice tool, and it has more icons for various network devices than Gliffy - https://www.draw.io/
