2001 Ford F-150 need help!

So I'm driving to base to go take a test. I notice that my battery light is on. I also notice that the farther I drive, the power meeter starts to go down to nothing. It shows on the meter that there is no energy in my battery. I park and cut off the engine. Then I go and take my test and come back a couple of hours later. I put the key in and it starts up without any problem and the battery meter shows the battery at half full. I continue to drive down the road, I notice that the battery light is still on. Eventually the meter starts to go down showing that the battery is emptying. The trucks engine goes off and I loose power steering etc. I pull into a parking lot.

That's where I'm at now. Not sure whether it's the alternator or a combination of things. Any suggestions are welcome.