but what will I be looking at as far as what line is what and which carriers I'll see?
The major spot in Joliet, as previously mentioned in this thread, is obviously Joliet Union Station. The station is situated at the crossing of the former Rock Island main line and both the former Santa Fe and Gulf Mobile & Ohio main lines.
The Rock Island trackage is now a single main track, operating East-West through Joliet, and owned by Metra until a point just West of the station. Joliet represents the end of the line for Metra's Rock Island District trains, which will either lay over and turn at the station platform, or turn to/from the coach yard located just East of the station. Moving Westward, track ownership is taken over by CSX, with trackage rights granted to the Iowa Interstate Railroad. A good clue regarding freight traffic is the 407 Bridge, easily in view to the West: The bridge is normally in the raised position for canal traffic, and is only lowered when a train is approaching to cross.
The BNSF Chillicothe Sub makes up the two North-South tracks closest to the station building and platform. The vast majority of traffic on this line is intermodal, as Corwith Yard more or less represents the end of the line to the North.
The former GM&O mains are the two running North-South furthest from the station platform, and closest to UD Tower. These tracks at the station proper are now owned by Union Pacific (by way of SP legacy), with newly constructed intermodal terminal Global 4 located approximately 5 miles to the South. Following the line Northward, though, these two mains change ownership approximately 1000 feet from the station, and from there begins CN ownership. Metra's Heritage Corridor (3 trains each rush period) and Amtrak's Texas Eagle and Lincoln Service trains (5 a day each way between them) utilize these tracks as well. (Note that the Texas Eagle is being detoured away from Joliet, though, until Wednesday 4/25 account track work downstate).
I hope this helps! Sorry, I don't have any idea of radio frequencies - not sure if you are scanner-equipped.