i put gas in my truck and shut it off when i got home. went to start it up 20 min. later and can't get it to start. any ideas

Does the starter turn over when turning key? If not then you might have bad battery or bad terminal on starter. If it cranks but it doesn't start turn the key as far as it will go without starting it and listen very closely for the fuel pump to kick in it may be hard to hear but if you have someone turn the key and you put you ear by the fuel tank under the car, you should hear the pump engage. IF you hear nothing, it could be fuel pump went bad or find out where fuse is to the pump and check that. If you hear the pump running then for some reason your not getting the fuel to spark. That would open up pages of discussion so just try all this stuff first

fuel pump is working. now what do i need to look for.

Well you either have no spark at spark plugs or else no fuel to intake. Unplug one spark plug and unscrew it from engine the plug it back in and having a friend turn the engine over, you hold the bottom part of the spark plug really close to the engine block without touching. You should see an arc going from plug to block. If not then you may have a bad ignition coil If you do have spark. You may have either a plugged fuel filter or a bad pressure regulator on the fuel line. Which u would have to test the fuel line with a pressure gauge with the fuel pump engaged to see what you pressure is at the intake manifold.