If you're not at least a little bit savvy about electricity, take it to a local auto electric shop and they can quickly diagnose the problem. If you are electrical savvy, use a DC Voltmeter across the battery with the engine running looking for 13.4 to about 14.0 Volts. If it's not that, do a Voltage drop test along the entire charging circuit. THEN, use a DC ammeter in series with the battery while everything is turned off to make sure there is no maverick current draw.