My maruti-800ac is creating lots of vibrations when it stands start,i hv recntly provided it to service hlp me?

when i start up my car,it appears that rpm get down significantly......in short-it just creat vibrations at any halt and feels uncomfortable....i have provided it to service ..i don't know what happens with my car..please give some reasons ....and help if u r getting what is the problem here..m waiting for reply thnxx a lot