Here
1. Check the fluid level in the current battery if possible. Top off with distilled water to just over the top of the plates.
2. Use a multimeter to test the voltage. Should be fairly near 12 volts.
3. Try trickle changing with a
battery maintainer for 2 solid days. Recheck voltage.
If that doesn't work (batteries are damaged by deep discharges) then buy a new battery from the list in the link. Other than the Optima, they are all generally the same so buy on price.
Some battery maintainers are also spoken of in the link. The one from WalMart is good enough and cheap. You want a maintainer as it is a trickle charger but smart enough to turn itself totally to "no charging" mode when the battery gets fully charged.