like fendor said shucks or napa can test it for free while its in the car, but ask lots of questions and maybe go to both places i've had shucks insist my batty was shot and told me i need to replace it, well a yr later it finally stoped taking a charge but mostly case the car was sitting for 2 months the battery finally died.
as far as batterys go, go to costco, they have the best prices i've seen and also have a lifetime warrty.
before you replace the battery take it in to shcucks and have them put it on thier tester, they also have a slow charge that sometimes can bring it back to life (in which case it maybe your alt.), and check the water level of the battery, at a yr old even a chepee should still work.