When I start my truck up it idles really high. It usually only does this when it hasn't been started in awhile like when I first start it up in the morning or if I go in a store or something like that for to long. I usually just wait for it to idle down before I go. I get it to idle down faster by slightly reving the gas and that gets it to stop sometimes. It gets kind of embarassing cause I go to work early and I live in a trailer park. I have an old truck so it is pretty loud and I have gotten complaints from neighbors they think that I am just out the reving up my truck to be cool.
You may have a bad IAC ( idle air control) but try eliminating any vacuum leaks and cleaning the throttle body first. Remove the intake snorkel, have someone hold the throttle wide open for you and scrub the back side of the throttle plate and surrounding bore with an old tooth brush and some carb cleaner. Be sure to spray some into the small holes next to the throttle plate. That should help stabilize the idle. If it still has a problem, replace the IAC.