Step 1: Sign Up
Before you can even think about 3D printing objects you have to sign up for Ponoko at http://www.ponoko.com/make-and-sell/pricing-and-signup. I would recommend to only use the prime service if you are going to print a large amount of objects/products using ponoko. Choose a unique name and password. After you have done this sign up for google sketch at http://sketchup.google.com/. It's really simple and fast to download.
Step 2: Open Sketchup
Once you have downloaded google sketch up double click the icon.
Step 3: Before You Start Drawing
I highly recommend watching these tutorials http://www.youtube.com/user/SketchUpVideo#p/p provided by google. They will give you more insight into how you can use sketchup.
Step 4: Draw Away
The best advice i ever heard when trying to learn to use 3D/drawing software is to "just mess around with it for a while." Use all of the tools and draw away. Get a feeling for what you want and try to execute it to the best of your ability. The first thing that I ever made using sketchup was a 3D blue blox with a mouth drawn on with skark's teeth.
Step 5: Save/Upload the File to Your Computer
When you are done making the object that you want save the file, but make sure to save the file in STL, DAE or VRML 97 format.
Step 6: Upload to Ponoko
Sign in to ponoko, check your stats and upload away. Then choose how long you want your order to take and order it.
Step 7: Take a Picture
When you receive your 3D printed object in the mail take a picture of it and put it up on ponoko's showroom to share/self-market and wait for the sales to come in.