Is there anyone here who has an agent? If so, how did you get one?
Ah. Then your talent will have to be spotted first. If you do a lot of plays or am in a reputable drama club, there will always be talent scouts on the lookout there. Or you can always go for auditions to show casting agents how good you are.
Unless you know somebody in the industry who can help you pull some strings, I don't think you can just pick an agent out of a catalog and hire them.
Where or when am I most likely to be "spotted"?
A talent agent, who could help you get onto tv