A vacuum leak is one possibility. More info on the vehicle would help.
you might have pulled a vacuum hose off
see previous answers, but if it idles high it might be the Mass Airflow Sensor. if your car idles too high for too long and you keep driving it, you could burn out the transmission module (very expensive )
idling too fast - adjust choke
I found that putting low grade gas in car mad it idle high. Does it every time
My 1990 idles at about 850 and with the ac on 1100
My car idles really high and wont go down and it will stall if I push on the gas!!
it should after it's started
the fuel mixture is to lean and it is bogging out the engin or u need the idle control to be tweeked a little
vacuum leak.
On my Chevy truck it idles high when its low on antifreeze but if its not that more than likely its usually a vacuum line loose or cracked. good luck.
vacuum leak
Broken/disconnected throttle cable