Hey there, if you want to set up a test game, you should head to
this page, which is also accessible through the "Games" link at the top of the site. There, you can click "Create a New Game" and that will lead you through the process. Then you can do all the testing you want in that game, including GM only options that aren't available in the general forums.
Otherwise, Koldik already gave you all the useful links you might want to check out, so all that's left for me to say is: Have fun! And if you have any questions, feel free to ask me or someone else. We're a helpful bunch here on GP :)