Go to Lowes or Radio Shack and get a cheap multimeter. Measure the voltage right at the battery with engine off, you should see 12 to 13 volts.
Start the engine and the voltage should rise to 13.5 to 14.5 volts.

If you dont see the rise, the alternator has quit working (as was mentioned above).

ALSO, look over battery terminals for corrosion, then follow the battery ground cable down to where it terminates in a lug at a chasssis bolt - mine was REALLY corroded there - I ended up replacing the battery ground cable.

If it isn't a bad, corroded cable, then Either your alternator has quit or your battery has failed.