I'd love to recommend you a free web application to create diagrams, but the good ones I know aren't for free, and you have to pay a cheap monthly membership.

UML modelling with Umbrello:

Edit /etc/apt/sources.list
$ sudo apt-get update
$ sudo apt-cache search umbrello
$ sudo apt-get install umbrello

For UML (and more) you can also use DIA. Here is the installation Guide ().