Why does my car want to shut off?

Okay, I have a 95 SE Ford Probe and its so loud while I'm driving it sounds like a motorcycle. The Rpm is High even when i first turn it on then when i start to drive if I'm not pressing the gas the Rpm gets really low and the car starts making loud noises likes its farting and wants to just shut off..but it doesn't..It used to shut off completely as long as i wasn't pressing gas but the person I took it to said he "accelerated it" whatever the hell that means then he said i need a injector and a new muffler but my mufferler is new!! and its Honda.....IDk if Im going to take it back to him because I don't think hes diagnosing my car right???? IS HE??? someone help me out plz cuz I need my car fixed asap.