Engine idles high when on park

This has happened a couple of times now, whenever I am sitting in my car trying to pull out of my parking spot or park, the engine starts revving up for no reason. I will not even come near the gas pedal and the card just starts revving up all the way to 10 rpm, or the number 10 on the tachometer. This worries me only because when I try to drive it goes lunging forward real fast, and the engine sounds like it's really being acelerated. Once again for no reason. I bought this car not even a month ago off of somebody on Craigslist, they were the second owners of the vehicle and told me they replaced the engine because the first owner treated the car very badly and the car had way too many miles on it, the engine has 90,000 miles on it and a guarantee of 100,000 miles. The last owner also told me I needed to change the front brake pads, I'm not sure if that is the problem, since I don't know that much about cars, but I really need to know why my engine idles so high on park or neutral.
Thanks