what the idle rpm for my 97 cav. (rpm for my car 900 when the car warm up ) is this high or noromal ? please help me .

mine is a 96' - 650 rpm @ idle

how can i decrease my rpm ?

first check to make sure that the cruise control cable and throttle cable are not preventing the spring from returning all the way on the throttle body. if the cables are loose allowing it to return back all the way, sounds like a bad sensor. (possible the idle air control valve, might try using carb cleaner to remove carbon from pindle)

You Should first check the Spec, is it and Auto or a Manual, Auto's will have a higher RPM normaly than a Manual. I have a 91 that idles about 900 also.

mine is also a 96' and an automatic

my car is Manual . ( how can i clean the idle air control valve)

it will be on your throttle body. there will probably be 2 star head screws holding it on. after you take it out, use a strong spray carb cleaner or any other solution to remove the carbon deposits and spray inside the throttle body where the seal is formed. when reinstalling, remember to push the piston all the way back into the sensor to prevent damange. the more carbon that is built up making a bad seal, the higher your rpm's will be at idle. if it's totally shot your car could idle as high at 3k.

oh.... ther is no oxygen sensor in my car is this the probelem for idle ?

no oxygen sensor? there should actually be 2 on your car. one on the exhaust manifold and one on the cat. converter. If it's not registering, you would have a check engine code reading o2 sensor failure or at least a fuel system too lean or too rich.

although my check engine lamp off .

unless it's a vacuum leak somewhere else, there would normally be a check engine code of some sort if a sensor or part was not fuctioning.