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.

DIA is an editor for diagrams, models, etc. Supports UML static structure diagrams (class diagrams), Entity-Relationship diagrams, network diagrams and much more. Diagrams can be exported to postscript and many other formats.

Let's start by opening synaptic package manager and searching for UML to install dia. Optionally, open the terminal and run:
$ sudo apt-get update
$ sudo apt-get install dia-common dia-gnome dia-libs dia-shapes dia2code

Output:
The following extra packages will be installed:
  dia gsfonts-x11
The following NEW packages will be installed:
  dia dia-common dia-gnome dia-libs dia-shapes dia2code gsfonts-x11

Resources:
Web: http://modeling-languages.com/web-based-modeling-tools/

For UML you can also use Umbrello. Here is the installation Guide ().