I'd recommend understanding synthesis. A couple of tutorials that may be of interest on synthesis:
You can of course jump straight in to getting to know how the ES2 works as this really really helps:
Steve H also has a section on creating Pads in the first chapter of the TNT3:
For third-party synths, check out Omnisphere,
...Absynth, Camel Audio products, Rob Papen products, Kontakt and there's really a whole wealth of instruments out there... dependng on what type of music you're interested in creating pads for.