Using a voltmeter is fairly simple and can really be a big help in solving this kind of trouble. So can a simple test light probe. Since the car is at a Walmart you should at least purchase a battery post cleaning brush and test light probe from them in their automobile department. Remove the battery terminals and clean them with the brush. Also clean the accessory power connection that Hellokit talked about if you see it.
Use the test light by clipping to a good ground point and then touch the probe tip to areas you want to test for power. If normal power is present the light in the probe handle will glow brightly. Use the probe to check the battery power while you try to crank the engine by placing the probe leads across the battery posts. If the light goes real dim then the battery is low. If the battery checks out ok then check for good power to the fuses under the hood. Some fuses may not have power to them until the ignition switch is on but I think most at least should be hot at all times.
I suspect the real trouble you are having may just be due to a low or bad battery. I recommend you have a set of jumper cables and a fair assortment of hand tools with you when you go to check the car. If the battery is really ok then there is a bad connection in the main power buss somewhere or the battery/chassis ground, so check that out. If you need a new battery then Walmart should have one.