Idle high and over heat

Need some help here.
It is scary to drive this thing, need this fixed ASAP.
In Park and Neutral, my RPM goes up to 3900 and sits there. Happens right at start up and rpm's never go down even when engine is hot.
In Drive it goes down to 1800 and sits there.
In Drive, if I let go of the brakes the car takes off like a bat, due to unusually high rpm.
When I drive this car, my foot is constantly on the brakes! Not good at all.

What is the matter with this thing?
Is it an idle issue?
A vacuum issue?
Something else?
Please help. It now over heats too.