In AO you have the option to choose one of the following sides/factions:

Omni- Tek
Clan
or just stay Neutral

I recommend, that before you choose a side, you do a little reading on the story behind the game, you can find it in AO's official site. Also here you will find more story.

To join a side in the ICC Backyard, you need to find the Recruiters, they are located in front of the building that is right across from the Unicorn Commander and the Field Surgeon Elsa Oosa. You can see it in this picture.

If you want to join Omni-Tek you should speak with the omni recruiter. To join the Clans speak with the clan recruiter, and if you want to stay neutral talk with the neutral observer.  The recruiters will give you an application to join the appropiate side, just right click it and choose yes when asked if you want to use the item.