Hello and welcome!!
To answer your question… yes, you can make content yourself from scratch for your Daz Studio figures. You will need to learn a 3D modeling program as HeraldofFire mentioned. He already mentioned Blender, but Daz3D also has a FREE 3D modeling program you can look into called Hexagon: (edit: I guess it’s not free anymore -sorry… but keep your eyes open…Who knows? It may go free again. )
You may also want to look into a 2D modeling program. There are several free ones that are pretty good. GIMP is quite popular and uses many of the brushes and add-ons that other popular paint programs use and is FREE:
As for where to find more free content, Chohole has mentioned some of the awesome Freebies that you can get here, but there are a few other places you may want to look at as well.
ShareCG - ShareCG has many freebies for various kinds of software:
RDNA- Runtime DNA is a content site that offers freebies. They have a few places to look:
http://www.runtimedna.com/ZeroPrice/ (will need to sign up to download)
Renderosity - Another content site for Daz Studio and Poser that offers freebies:
Hope this helps a little!! Good luck and Welcome again.