Kicad is the software we use for designing PCBs.
For linux, just run
sudo add-apt-repository --yes ppa:js-reynaud/kicad-4 sudo aptitude update sudo aptitude install kicad kicad-library
For Windows,you can download it here.
Below is a video with a short introduction to KiCAD.
On linux, when you try to open up stuff, it may say you're missing a ton of libraries. To fix this, launch EESchema, then go to “Preferences→Component Libraries”. Add `/usr/share/kicad/library` to the user defined search path.