1) You just hit level 20, congratulations! Automatically right as you hit level 20 you get a quest in your quest log called 'Factoids', which asks you to complete another quest called 'Study'. Anyways go talk to a man named Rio. All you have to do is open up your quest log and click the green name 'Rio' He will give you a quest called 'Study'.
Picture of Rio (He's located in Freedom harbor, near the center):
2) Now that you accepted the quest, it asks if you can go to the library in freedom harbor to talk to 'Niachilo' about the quest 'Study'. Once you talk to Niachilo in freedom harbor all you have to is return back to 'Rio' and you will have completed the quest and received the wonderful level 20 mount.
note: other mounts in the game you can get by obtaining a higher level and by having a certain reputation, which I will update the guide to help people with as soon as I get there.
Here is a quick video
by visch