1) Look at your gas gauge. Even though it's obvious we need gas in our tank we sometimes forget and let it run out. Check to see how full the tank is. If you are unsure and think maybe the gauge is off, try adding some gas to the tank from a gas can and then see if it will start. This will save you a lot of time and is easy to do if your gauge is not reading correctly.
2) Check for fuel at the engine. You could've done this first but it's very easy to check the gas gauge and we want to take the easiest steps possible first. Since most vehicles don't have carburetors anymore we have to check pressure, if you have a fuel pressure tester that you can screw onto the fuel rail, that is the best way to go. If you don't then you can cautiously remove the little cap that covers the test port and use a small screwdriver to push in on the valve to see if gas is present. This won't tell you pressure but will give you and indication that gas may not be the problem You can try spraying some starter fluid into the air intake,and crank over the engine,if it attempts to start and run you will know lack of fuel is the problem.Listen for the fuel pump running when you turn the key on,they often fail. You can try rapping the botom of the fuel tank with a block of wood or ruber mallet as a helper cranks over the engine,this many times will get a stuck pump to work,although most times only for a few minutes. If the truck starts and runs even for a few seconds you will know the fuel pump is your problem.If none of these steps give you ant results continue on to the next step.
3) Check for spark., get to the spark plug wire and remove it, you should be able to do this check. Take a small screwdriver (a known good spark plug works even better) and insert it in the spark plug cable and then lay it close to a ground. Have someone try to start the car and watch to see if you get an ignition spark that jumps to the ground. If you do then you have eliminated the wire, coil and ignition system from being the problem.
4) If you don't get a spark at the end of the cable then go to the coil cable and check for spark there. If you get spark at the coil then you may have a distributor cap problem if applicable to your car. If you have no spark at the coil then you may have a crankshaft sensor or other computerized problem. You will most likely have to call the repair shop and have them come out to look at it or have it towed.
For an engine to start it requires fuel, spark and timing. Without one of those three things the engine will not fire and run. Always check the simplest things first and try to eliminate several items with one check such as checking at the spark plug wire cable before the coil.