I am unfamiliar with P.A. Systems, and I want to hook up my band's amps to the P.A. System at the show we are playing at. What kind of cable (or rather cable-end) do I need to do this(i.e. do P.A. systems use jacks other than 1/4 inch)? (will a regular guitar cable with 1/4 inch plugs on both sides work?)

Also, my guitar has a pre-amp send and a speaker out slot - which one do I use to hook my amp up through a P.A. system? I know the speaker out slot is to make my amp go through a cabinet if I ever decide to get one, but is it also to hook up to a P.A. system? Each of these slots has a 1/4 inch jack. Thanks for the help.
OK you say amps so I assume you mean everything, guitar and bass plus anything else you play. You have two ways of doing this micing up the amps which will retain the tone of the speakers or DIing (direct injection) which will send a clean signal to the PA.

Using a mic is obvious just put the mic a few cm away from the speaker and run it to your mixer.

To DI you have the choice of a mic level signal or a line level signal. For a mic level signal you need a DI box which you plug your guitar into and then run one lead to your amp and the other to your PA. Get the shop to explain how these work as there are different ways of doing this. You might even take the output from your effects.

Line level DI has the advantage of using the tone settings of your amp so it will be a bit closer to the sound you are used to. Many amps have a DI out socket so just buy leads to match. If not you can use the pre amp send though it may be wired to disconnect the amp when you plug in. You can also us the effects (fx) send. Line level inputs normally use a jack lead.

In no circumstances use the speaker output directly to your PA unless you want to replace them.

You need to start reading about PA, start here.
Don't DI your amp. They sound infinitely better through a microphone.
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If you're gigging often, it's worth buying a microphone. I'd suggest buying a small stand, decent XLR cable and everything else you need as well.

Put a coloured tape on them so that you know it's yours. I've had SO MUCH stuff stolen at gigs.
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Stick a Shure SM57 in front of your amp's speakers. About the only thing I'd DI are keyboards and maybe bass (if you want dead clean bass) and if the bassist is running something good like an Ampeg I'd stick an SM57 in front of that too.
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I'd pretty much agree with this. Your cab is usually so much of your sound for a guitar that getting the same tone out of the pa is tricky. The Shure sm57 is a great instrument mic and is the one to beat it costs about £80.

DI bass is usually fine but you need a good pa to handle the bass. If it is underpowered in moderate venues it can be better to leave it coming out of the backline although this can make it awkward to get a balance.