On some vehicles including my truck it's normal for the engine to rev at a bit higher speed for a few minutes so that it warms up.
Then after a minute or two the engine slows down to the proper idle speed.
So if your engine revs high for just a few minutes and then goes to normal idle speed then that is usually normal.
But if your cars engine continues to rev high after starting it then it could be caused by several different things such as a vacuum leak, intake manifold gasket leak, throttle plate stuck open, cruise control stepper motor stuck in one position; faulty throttle position sensor, and/or faulty idle air control motor or solenoid.
Your engine when revving high is sucking in too much air which causes the engine to speed up just like when you depress the gas pedal more air gets introduced into the engine which increases the engines RPM and when you let off the gas pedal your engine slows down because of reduced air flow into the engine.