You need a battery that:
1) fits in the allotted space
2) the + & - connections are oriented so you can attach the cables (you may have a little or a lot of freedom on this; a lot if you are willing to make up longer cables and have the room for installation
3) you want something labeled "deep cycle." don't get some combo battery that also says "starting," and/or shows a Cold Cranking Amp (CCA) rating as that is not a true deep cycle battery.
4) you can get two (or four or six) 6 volt batteries and wire them together to produce 12V battery "bank," or you can get 12V batteries. There are plenty of arguments for each. Your end result will probably be determined way more by how you treat the battery(ies) than by their initial quality practically regardless.
5) after satisfying the above, bigger is better. You are looking for Amp-Hour rating (AH). There are tall format batteries (fit in a given footprint but need the height to accomodate case height & terminal clearance) and batteries that pack more AH in a smaller case. If you know nothing about batteries and want the most simple criteria- weigh the battery options, literally. Take a bathroom scale & get the heaviest, which will contain the most lead.
6) get a good 3-stage charger/maintainer and keep the battery properly charged at all times when not discharging it. Don't over-discharge (cycle it down too much). Good deep cycle batteries will work fine down to 50% charge but then need to be charged back up.
7) if you want a better education to manage 12V intelligently, the 12 Volt Side of Life
is a fabulous write-up giving great info and insight. Read both installments (link to 2nd part is at bottom of that page which is Part 1).