Why is my engine revving itself?

I have a 1998 Honda accord. If I go out to start it, it will be fine but after a few minutes it revs itself while in park. I put it in drive or reverse and it stops. I'll be on my way to work, get into the parking lot, put my car in park and immediately it revs itself. I know barely anything about cars haha so any ideas would be awesome!

Idle hunting is a favorite Honda hobby. A common cause, oddly enough, is a bubble in the coolant. There's a bleed nipple on one of the housings where the radiator hoses attach to the engine. When the engine is cool, open the nipple 1/2 turn and close it when a steady stream of coolant comes out. Of course, be sure the radiator is full of coolant.

If that doesn't settle it down it is a job for a Honda specialist. Many people try replacing the IAC valve but that rarely helps.

Probably a Vacuum Leak… Check all your hoses and fittings. If it's not that your IAC or Throttle body gaskets may be bad or just loose. See if you can tighten any of the fasteners.

Check the throttle position sensor. It may be bad. I had that same problem exactly how you described it on a Hyundai Santa Fe. A $10 sensor replacement did the trick.
now just as a note, my problems started very lightly. At first it would go up to like 1000 RPM, then back down. Then 1200 RPM, eventually it would climb all the way till 3000 RPM and putting it in drive was a really harsh thing to do.

the sensor replacement is actually quite easy and you can do it yourself. Just gotta buy the sensor. That being said, before you drop any money into this, try out FlagMichael's fix before just in case it works. If it doesn't then try mine. Even if it isn't the throttle position sensor, you are taking a $10 gamble. If you lose and it in fact isn't the sensor there's not much harm done.

I would take off the intake manifold and clean it and then put it back on with new gasket…