First, the no-cost check. Make sure the clutch is correctly adjusted:

- play at the pedal
- play at the clutch arm under the car
- No thick carpeting under the pedal
[edit] See the tune-up article for the clutch adjustment specifications

If you have more than an inch of play at the pedal, you won't be able to engage the clutch all the way. Hence gear-shift problems. If you have no play underneath, the clutch will slip.