First thing you need to do is check the charge output from the alternator, you will need a multimeter or a voltage gauge, you can buy a reasonable one from your local parts shop, check voltage at the battery posts, should be at least 13.5 volts upto around 14.5 volts, if it is any less than 13 volts you then need to check it at the terminals, if it's still less than 13 v you will need to first check the alternator belt (is it tight and not slipping?) if thats ok, then you can be fairly certain you need your alternator repaired. with the multimeter you can also check all grounds, check from batt - post to eng block, then check from batt - post to body (find a bolt or body ground mount) with eng off, key out and all doors shut you should get a low reading of around 1-6 ohms resistance, if it's any higher you need to fix/clean all grounds.