@UntitledWater:
It's pretty easy, if you have the battery as you say so you don't need DC Converter or stuff. But if you wish to charge your battery at home only directly connected then you will need DC Converter that also of same or more AMPS as of your battery.
Anyhow, the way you should connect is simple.. in your Amplifier there are 2 wires. +ve & -ve (referred to ground)
+ve Amp wire goes to +ve terminal of battery
-ve or ground amp wire goes to -ve terminal of your battery, in CAR the -ve terminal is connected with body, so whole your body of car has -ve terminal access.
the Remote Wire that comes from the amp is usually for wired-remotes or bass control knobs. or if you have expensive amp that should be wireless
Same method should be adopted for connecting your Head Unit of car if you need that, otherwise Amplifier has Pre-Input which will be connected from your Line-Out of your PC to your Amp's Pre-Inputs.
btw, if you feel there is problem post your Amp picture where the terminals are connected maybe somehwere can explain along with in much details if provided picture