What you are looking to purchase is a electronic battery isolator kit. What it does is allows you to add an extra battery which stays separate (isolated) from your main battery. When the vehicle is running, the isolator charges the primary battery first, then when it's full, it will charge the auxiliary battery. When the engine is cut off, the auxiliary battery is kept separate and allowed to power your extraneous devices, like your entertainment system, without draining your primary battery. There is a side benefit to doing this, as with a flip of a switch, the auxiliary battery can be used as a power source to start your vehicle. Here is a YouTube Video which explains it very well.
I don't know enough about them to recommend any specific isolator, but this guy has a pretty good write-up about the Enerdrive VSR system. I am not recommending it personally, but would suggest you get one with similar attributes.
I believe you'll need the isolator, some cabling (you'll have to figure out which gauge you'll need - also remember that the longer the run, the larger gauge you'll need to compensate), the battery (which you already have), battery tie down/mounting point, and connectors. I'm sure there are plenty of kits which you can purchase which will have everything besides the battery. If using a standard lead/acid battery, you do not want to mount it within the cabin of the vehicle, as it produces gasses when charging/discharging which can be hazardous.