Have the battery checked first, if the alternator isn't charging it's probably going to test low but may just need a good charge to bounce back. If it's more than 4-5 years old you may just want to replace it at this time.

You can check the charging system using a multimeter set on DC Voltage right at the battery posts. A healthy charging system should be in the 13.4-14.4 volt range.

While you have the multimeter out this is a good time to do a voltage drop test. This quick test is important because it checks the condition of the main grounds on the engine/body and on particular circuits..

To perform the test set the multimeter on DC volts and read the voltage at the battery posts with the motor off. Remember this reading. Then take the black lead off the battery negative post, and place it solidly on the engine block or cylinder head, keep the other lead on battery POS Terminal. Take the reading again and compare it to the first one. They should be the same -or very close- if the grounds are good.

Example:
If you got 12.7 V at the battery posts and 12.7 V on the engine block/cylinder head you have excellent grounds.
If you have 12.2 V or less at the engine block/head you have a bad ground or cable.

The standard was any loss of .5 V (1/2 V) or more is a problem, but with the amount of electronics today I condemn any ground that shows .4 V loss or more.

You can also test against the body by moving the black lead on your multimeter to the strut tower or to any ground straps or bundles that you find on the firewall or anywhere else on the body or engine. I check them all, it's quick and it's often overlooked.

First check all the connections. If they are tight and everything looks good then take it to a parts house. They can check your alternator and your battery. If you have a battery charger, charge it up before you take it in. That way when they load test it you are getting an accurate reading.