This part is simple. You need to have established an account at Shapeways. Registering is free. Once you have the account, then select MyDesigns/Upload.
Use the Browse button to find your file. If you did make an STL, then Shapeways will ask you for units. You want to pick millimeters! Selecting "meters" will make your model 1,000,000 times more expensive!
Pick a descriptive title for the Model, and any Galleries you'd like to connect to.
It's at this point that you must chose whether to sell your models to other people or not. If No, then select an Availability of "Only for Myself", otherwise, pick "Available to All"
Consent to their terms & conditions and click Upload. You will get an email once the upload starts, and another once their software has checked a few things and passed the model as printable.
Click to open Shapeways website.
Step 11: order, paint and enjoy