My car won't accelerate past 20 mph but the motor is reving?

I have a 2002 Honda Civic and when I try to accelerate past 20 mph it won't go anymore even when I hit 6 rm, doesn't happen at first but when I stop and try to accelerate past 20 again it won't accelerate anymore but when the car is first started it will.

Slipping clutch.

Yes,
motor will be reving

The clutch is shot.

Well if the motor is revving, but the wheels aren't turning you've got to to ask yourself what is between them: clutch, transmission, cv joints… Assuming that the driveshafts or wheels haven't fallen off, that leaves the clutch/transmission.

New clutch needed.

Assuming it's a standard transmission, the clutch is shot. Depending on how you drive the clutch needs to be replaced every 35,000 to 50,000 miles.

Time for a rebuilt transmission.

Manual- clutch is shot. Automatic- too bad, too bad, that generation got a bunch of lemon transmissions that fail every 30,000 miles. You can have it rebuilt, but the problem still recurs the next 30,000 miles. Lookee here:

http://www.hondaproblems.com/trends/transmission-failure/

TRANS TYPE! If MANUAL a bad CLUTCH! IF AUTOMATIC same answer but MORE expensive! CHANGE TRANS FLUID! Have THROTTLE BODY and IDLE AIR CONTROL VALVE cleaned up to SHOWROOM! POWER FLUSH your brake and CLUTCH fluid with FRESH factory ONLY fluid from HONDA! BRAKE and CLUTCH fluids are ONLY good for 2-3 years tops on ANY car! Then it tuns into GOO< TOXIC WASTE and WATER LOGGED tar! It MUST be POWER FLUSHED every 2-3 years unless using NEWER SYNTHETIC EUROPEAN or JAPANESE fluids!<<<MANY PEOPLE even PAY for All NEW clutches, etc when ONLY a BRAKE FLUID change is needed! BYE NOW and send me the CHANGE!