If I was to sign myself up to an agency, what sort of payments should I be expecting? Upfront fees or is that a scam?
I figured that you would just have to pay for head shots and studio hire, but some speak of fees that are hundreds of dollars just to join, and I don't know what they cover.

Can someone give me a run down? Thank you!

Superhigh upfront fees, especially with no explanation? Scamtastic.

They're supposed to be your agent or manager. You're the client. Why would you ever need to pay before they get you jobs? If an agency believes they can make money working with you, then they should be making an investment in you, not the other way around.