Need help my car wont start

What do you mean by "dies down", the engine stalls? Do you have a digital voltmeter? If so, measure the battery voltage while the engine is running, then holler back.

I get the impression you have a charging system problem. In addition to that, since you had to disconnect the battery to replace it, the Engine Computer lost its memory. It is going to have to relearn "minimum throttle" before it will know when it must be in control of idle speed. Until then, the engine should stay running if you hold the gas pedal down about 1/4".

To meet the conditions for the relearn to take place, drive at highway speed with the engine warmed up, then coast for at least seven seconds without touching the brake or gas pedals.

The engine should run for at least a half hour on the new battery. Once minimum throttle is relearned, or have a helper hold the gas pedal down a little, use an inexpensive digital voltmeter to measure battery voltage while the engine is running. It must be between 13.75 and 14.75 volts. If it is low, suspect worn brushes in the alternator, but we'll need an additional test to confirm that.

I'm suspicious you need an alternator but we aren't going to replace it until testing proves that is the problem. What happens with the engine when you hold your foot on the gas pedal?

You have to keep holding the pedal down. You might have to drive with two feet; one on the brake and one on the gas.

Sure. All that's happening is the engine is idling too slow and you're making up for that by holding the gas pedal down just a fuzz. It isn't exactly fun driving like that but once you're on the highway and you've coasted for seven seconds, it should idle fine.