Did you try the crankshaft position sensor like the previous guy asked? I had a jeep that exhibited the same signs as yours and that was the problem. It could be running fine one day, you shut it down, go back out the next day and it won't run. These sensors tend to get soaked with heat and short out when they're cooling down from a run session.
I'm impressed that you're so young and gung-ho about repairing this car yourself. If I could make a suggestion: go buy a book on the car, and one on fuel injection. To be perfectly honest, there are many components, but the outputs of the sensors are very simple. Usually most of them just send "yes or no" signals to the computer. Don't be afraid to crack open a book and learn about this stuff, the knowledge you gain may help you to assess your problem more carefully.
Your problem could be any number of things. From the above however, I respectfully disagree with the cat/o2 sensor. Generally a cat will cause problems at RPM/create smells. O2 sensors may cause the car to run poorly, but I've not seen one cause a car not to run.
The reason I asked you to check your fuel pressure is that on fuel injected cars the fuel rail has to be at a set pressure for the injectors to open and fire. Generally most of the setups on regular driver cars are "batch fired" which means many injectors fire at one time, and the charge just sits in the intake port until the intake valve opens for the cylinders that aren't on the intake stroke when the batch goes off (less dramatic than it sounds, we're talking a fraction of a second). What it does mean, however, is that the fuel rail is more sensitive to say a somewhat plugged fuel filter, because fuel is drained in pulses rather than steadily. Therefore the fuel pressure needs to be adequate and steady.
Do some reading, its obvious you have the drive to fix this problem.
K