Car idle's crazyyy pls help!!!?

hi I have a Holden barina 1990 auto and everytime I stop at say a red light my car will either rev really high or really low to the point I have to hold my foot on the brake and accelerator to keep revs up or car will conk out! How can I fix this problem and how much would it cost australian dollars? thankyou for your time... Cher :-)