Your first step is to download and learn a CAD program, a few popular / free ones exist.
Diptrace (Pin Limit)
Eagle (Size Limit)
KiCAD (Completely Unlimited and open source)
MultiSim Blue (Mousers' offering of NI MultiSim, pin limited)
RS Design Spark (Completely free / unlimited as far as I know)
There are probably others which i've missed, but some people have success with a varied number of packages, Eagle and KiCAD are natively supported cross platform, not sure about the others.