Car idles low or stalls.

What is your question?

Car has started to idle very low, like 500-750 rpm low. It ran fine up until I took a trip of about 120 miles. The very next day when I start it up the RPM's jump to 1500-2000 then immediately the engine cuts off. I have to have my foot on the gas to keep it running but still idles lower then normal once the engines warmed up I can stop without it stalling but it will idle at 500 rpm. What could be causing this and how do I go about fixing it? Thanks in advance.