When i first start my car the idle is very high but goes down after a few minutes

Engine idle should be higher during "open loop" - when the engine is not yet at operating temperature. Then, when it reaches "closed loop" - when the engine is operating at over 190 degrees or so, the engine's computer relies on data provided by many different engine performance components to operate the engine far more efficiently, and the engine speed will lower. However, if the engine speed fluctuation is excessive, there could be an issue with fuel / air mixture ratios that may need to be addressed.