Throttle plate

I am having trouble with my acceleration, and the car stalls after warming up on idle. A friend told me it could be the throttle plate, or throttle bypass, or even the throttle sensor. I trust his judgement since I have been to 3 mechanics over the past year and noone could fix the problem(tried to say it was the computer and would gladly replace it for $500.00+) I just need to know how to do this.