Okay, This is kind of big topic and rather a general question and not really Daz Studio Specific.

So I'll have a shot and try to get you on the right track.

I'm not overly familiar with sketchup and its export of the obj format.

But..

Your primarily going to need to have your obj file UV mapped so you can begin working on your texture files. I believe UV Mapper should be able to do that for you if your model isn't already mapped.

Though iIt sounds like your obj has material zones and is probably UV mapped and ready to start creating textures. To begin you will need to have saved templates of your UV's.

Again UV Mapper should let you do this. You need to be aware of overlapping sections in the UV map and other potential issues. 

To test in Daz Studio how your UV Maps are looking I'd suggest apply an image like the below image. Sorry about the distortion in the column preview its resizing and should be square.

Applying this or a similiar map will let you see how your UV's are working or if there is any stretching or strange behaviour. You may need to repair or rebuild the UV's.

Strong UV mapping helps to create good texture maps.

Using UV Mapper export a UV texture template you can open in photoshop it shoukl look something like the below without the descriptive text.

The next thing you will need to consider is the resolution requirements of your texture maps. Depending on how you look at it, but for a 3d still render a 1024x1024 pixel texture maps is fairly small. The size is dependant on the render requirements of the object. Generally the closer the camera the larger map will be rquired to retain resolution integrity.

The above texture works with the following geometry.

The geometry is split into a number of material zones using the same UV map. The following image shows the geometry imported into Daz Studio 4 and I have applied the UV template to the Diffuse channel and the opacity channel in the surface tab.

 

The Genesis Barrel ring uses Daz Studio Shader presets for the gem stone and UV Texture maps for the ring surfaces. The main textures you will need to create are a diffuse map for each are and a bump/dsplacement map. This gives you color and depth. Other maps you may require are Specular, ambient, reflection and opacity/transparency.

The following render uses numerous maps built in photoshop from the UV Template. And the gemstones are a Daz Shader preset.

 

I hope this is of some help and gives you a little info. Feel free to ask more specialised questions and I may be able to help you out more.

See more of the Genesis Barrel Ring here

Sorry for any typos or mistakes its getting a little late here.