When you'd like to start building more complicated objects (http://reprap.org) with your printer the dimensional capability becomes more important. The spacing between holes for instance may determine whether you are able to bolt it to another part.
Printing a calibration block is a good way to get your feet wet on a printer you haven't used before and will set a good foundation for future printing, knowing that the printed size is correct.
I made it at TechShop!! The Detroit Tech shop (http://techshop.ws) location has a Makerbot Thing-O-Matic and will also be setting up a new Makerbot Replicator with dual Extruders in the near future.
Step 1: Selecting a Calibration Block to Print - The Universe of Thingiverse!
If you go to the Thingiverse home page (http://thingiverse.com) and type "calibration" (no quotes necessary) into the search bar and hit return you will get a long list of calibration and test objects to select from.
Depending on the type of objects you plan to print after your calibration check you may want to select something that offers thin walls, overhangs, bridges, or holes. But my suggestion if you are just getting started is to chose one that is simple, but will give a good indication that your X, Y, and Z dimensions are working correctly.
I selected the Calibration block by kludgineer (http://www.thingiverse.com/thing:4203) which provides 10, 20, and 40mm lengths in all three primary axis to check. This will help determine if any issues are due to printer scale or if they are related to other issues such as material expansion.
You can download STL files from Thingiverse by selecting them from the LH column of Thingiverse item pages. I've also incuded Kludgineer's Calibration Block instructable (as of 8/11/2012) under the sharing terms allowed by the Creative Commons GNU/GPL license. I'd encourage you to always check the Thingiverse page for updates, particularly on more complicated projects.
Once you have your STL file downloaded to your local computer we will get ready to print it.