If you plan on making actual original 3D models (completely different apps than photoshop), it'd still be worth it to learn from drawing studies. That way you'll have a basis of proportion and use of volume to go from. (The concepts still apply regardless of method.)

Since you claim to be good at figure drawing, then using a tablet might help if you don't have one already. Wacom appears to be the best in the market. (They pretty much own it really.) Then you just draw as usual, using color and lightness/darkness to produce a 3D appearance as you would with traditional media. (There is no one right way, as it's a matter of practice, understanding of lighting, and stylistic approach that can achieve the 3D look via drawing/painting.)

Now if you want to make an actual virtual 3D object, that's a different issue. In this regard Photoshop is only good for texturing, making reference teplates (plan drawings), or postwork (touching up/editing a render) as it's a 2D raster program and not an actual 3D modeler/renderer. For better info and guidance in this area, you should ask at the 3D modeling forum. You'll find plenty of help there if that's what you're looking for. From there you'll get direction on starting out and what apps to get in regards to a workflow and budget that suits you. (Personally, I'd suggest Wings3D to a complete 3D modeling newb. It's free, has what's probably the easiest interface/workflow, and keeps you focused on the basics of subdiv-style modeling which are applicable to many far costlier applications.)

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Barbequed Pixels?

Your friendly neighborhood Wings3D nut.
Also feel free to browse my freebies at ShareCG.
There might be something worth downloading.