I'm assuming it's turning over (when you turn the key, you're hearing the rrr-rrr-rrr sound and not either click-click-click or nothing).
Most likely you've got a bubble in your fuel line that is preventing the fuel from getting to the engine. There's probably a valve on or near the engine that will let you bleed the air from the line, but without knowing the specific make and model we can't tell you exactly where that would be. It's USUALLY on top of the engine towards the firewall, and generally looks a lot like the air valve on a tire.
To bleed the line, call a mechanically savvy friend. If she can't find the valve either, you may need to go to an auto parts store and buy a service manual for your car (should be less than $20 or so).
Check spark plug wires, and voltage on the distrubutor.
The problem isn't your batter or alternator or starter then. Start looking into your fuel management system. Possibly your fuel pump, injectors, etc.
Voltage Regulator?
It could be the starter in it. It may be weak or need replaced.
it may be the starter going out doing it . it will kill the alternator if it keeps going . I know i had it happen to my 96 olds replaced the alt still didnt start and the starter fixed it
I had a similar problem turns out the Alternator was bad.
the battery job is to start the car so the alternator will kick in after the car is running
Find an ammeter to find out what the starter is pulling. If the starter is pulling too much current, you probably need to replace the starter, but don't just go off replacing parts until you test them. Apparantly that plan didn't work to well for you when you replaced the battery and alternator. Always test first!
Start the truck then disconnect the battery if it's still running the alternator is good
Check your alternator and your connections form the battery to the Starter. You may just have a bad connection If that does not work, your solenoid on your starter has gone bad, a new solenoid or starter may be needed. Check the start switch on the clutch pedal if so equiped.
Starter motor problems, charging system problems, wiring and connection problems between battery, alternator and starter. To many possiblities to list. If the battery is still charged and you need to jump the car to get it to start you have a bad starter. Now you say how can that be? it starts when its jumped? Correct the starter is drawing too much power"amps" and having a second battery "the jump" appx doubles the amount of amps to the starter allowing it to start. Replace the starter and problem will go away. Besides it wont take too long before a jump wont allow it to start.
Did you replace the battery? I am trying to determine why you changed out the alternator which charges the battery, and the starter which only starts the car. The 2 are only related through the battery. One system does not rely on the other. As you drive the alternator puts a charge out to recharge and maintain your battery. Your starter only engages when you turn the key to start the car and this draws a lot of voltage from your battery. Try to explain in better detail: Like the car will try to start, it will start but won't keep running or the car won't even turn over, or I am getting a clicking sound. The better you describe the situation the easier it is to get you pointed in the right direction