First off, what year is the vehicle and what engine? Have you pulled the codes?
It's common for the engine to rev up to 2000 RPM and stay there for a little then calm down to idle after starting. If you press on the gas pedal and raise the RPMs yourself it should then end the high idle process and go to a normal idle. That part is all normal. There are a few things that can cause a screwy idle. First is the IAC (Idle Air Contol) as mentioned by Juan. The second is the EGR valve not fully closing, the third is a dirty throttle body.
You can force the EGR to close by pressing down on the diapragm by inserting a screwdriver through one of the holes and pressing down, or remove the sensor on top and pushing down. Disconnect the vacuum line and plug it and try idling the engine. I
As for the throttle body:
I don't know how old the vehicle is and how many miles are on it, but a dirty throttle body is a pretty common thing. What happens is carbon builds up in the throttle body and prevents the plates from fuly closing. Cleaning it is pretty simple. What you will need is: A new throttle body gasket, a can of throttle body cleaner (DO NOT USE CARB CLEANER!), and an old toothbrush. Removing the throttle body is very simple. Disconnect the throttle linkage from the top, and if an auto tranny the kickdown cable. Remove the two hoses coming in from the air filter top. Disconnect wiring to the IAC and TPS (Throttle position sensor). Disconnect the two vacuum lines on the side of the throttle body behind the IAC going to the charcoal filter. Disconnect the two coolant lines. Remove the four bolts holding the throttle body on to the upper intake manifold. If the coolant hose doesn't want to co-operate I usually just cut them and replace them. The throttle linkage removes by pulling it straight up, the kickdown cable for the auto tranny is removed by pulling it forward then up.
After you remove the throttle body you can remove the IAC from the side of the throttle body and the TPS from the bottom of it. Lay some newspapers down and proceede to hose down the throttle body. Use the brush to get into the nooks and crannies and get all carbon out. You can remove the throttle plates with a phillips screwdriver. Just make sure you put them back in the same orientation they came out. The whole removal, cleaning and reinstallation process takes less than an hour.
When you put the TPS back on, make sure you have installed it correctly as it designed to work in one direction only. Don't forget to reconnect the two vacuum lines!
Glenn