OP,
First thing is to verify that the battery is good. A new battery exhibiting signs of failure, or even being DOA out of the box, is not unheard of. Verify that all of the connections are clean and tight. Verify that, if the battery is not maintenance free, that it does not need water. With the engine off, test the voltage across the battery terminals. It should read about 12.5 volts. If less consider a bad battery. With the engine running, again test across the terminals. The reading should be about 13.5 volts or more. If less post back with the result.