This works sometimes on jeeps.
Remove key,disconnect battery,turn key to the run position(not trying to start it)
and then connect battery cable.
If alarm goes off,remove cable and connect again.
If you connect battery with out alarm going off,then it might start
and stay running.
It is the ignition. You either changed the ignition and your car still needs the chip from your old key or you had a key made with out the chip and you need a chip in the key. You can normally get them at a dealership but they are around or over 100 dollars.