poor gas milage how to fix?

how do you increase gas milage only geting 10 miles to a gallon

engine lite on full sometimes
Engine lite flashes after driving

Usually after it has run awhile and you stop some where and turn the engine off. when you start again then the RPMS drop to around 5-6 hundred every time you come to a stop.AC will not cool properly. Turn AC off and drive awhile and problem usually stops for awhile.

My radio will be playing fine then the sound stops as if the signal is lost. Without doing anything it will start playing again. The antenna looks OK.