How to buy a battery?
LOL, no one has answered your question directly.
Basically, first, you are going to find out what the physical size the battery is.  This is generally referred to as the "group".  It can be Group 24, 34, 26, etc.  That refers to the dimensions of the battery itself, not the electrical power it contains.
Next, you have three choices.  
1. Flooded lead/acid - they are cheap, common, and relatively inexpensive.  If you choose this one, buy the biggest CCA and RC (Cold Cranking Amps and Reserve Capacity - listed in minutes) you can afford/find.
2. AGM (Absorbed Glass Mat).  They claim to last way longer, can be mounted in any orientation, and generally have more power.  Optima is a common brand here, but there are others.  Look for high CCA and RC.
3. Li-Ion.  Litium batteries for automotive applications are just coming out.  They are much lighter than 1 or 2, will provide a LOT more power, but are really pricey.
Good luck!