Just play around with it, and practice as much as you can. Explore all of the different things you can do, and also some plugins.
Also play around with materials and lights. I've never used premade lightrooms and materials. And imo, they are not essential for good designs. My lighting is generally very simple, I just throw in a few omni lights and crank up the brightness a bit.
As for materials, try using solid colors and play around with specularity, reflection, etc.
For textured materials, make sure you use tileable textures, set the mapping to cubic (for best results imo), and use normal/bump maps. Normal/bump maps with drastically improve the quality of your textured objects as the textures will react with the light, and won't appear flat.
Finally, a lot of the magic of 3D signatures happens in Photoshop imo. So it's good to collect loads of different light textures and effects and learn how to properly use them.
Anyway, enough of my rambling. Overall, practice practice practice. Try offering some free graphics for a while so you can get some ideas of things to work on.
