I have an agent, how and where do I find auditions, do I use "backstage"? If so, what do I do?

Try a local Amateur dramatis group , places like that .

They are keen for new people to join .

Read the tips on this link:
http://www.backstage.com/advice-for-actors/backstage-experts/4-things-make-kids-stand-out-agent/

You become an actor by acting.

Start with your local community theater and/or area theater troupes. Audition often, even while you're not very good. Ask actors that you see at those auditions (especially those who are good) where reputable acting classes are and then take them.

Study good actors. There are online master classes from actors you can watch. Take an improv class (or two).

In other words, treat acting like any other art form: study, practice, work.

There are company's on the web that look for extras for films etc

Acting is NOT about being loud. Extroverted people *tend* to land more roles because they are willing to take risks and they put themselves in lots of auditions. Over and over and over again.

If you want to learn how to become more comfortable auditioning and acting in general, I recommend an improv class or two. Beginner classes are specifically for people with little to no experience on stage (at local improv theaters).