Testers are free to create both Asmodian and Elyos avatars, both of which will enjoy substantially increased XP gain, crafting, gathering, and item drop rates, and Abyss Point gain. Accessing the test server requires a separate Aion installation on your hard drive, which can be accomplished by duplicating your existing Aion folder or via a fresh install.
NCsoft cautions new testers that character wipes may happen at any time, so don't expect the PTS to feature a persistent version of your main with increased XP. Test server characters also will not count against your character number limit on Aion's live servers.
Check out the installation instructions as well as the technical discussion thread on the official boards. Aion Source has also dedicated a forum to the public test server.