Are you having problems engaging gear when stationary? If so, pump the clutch pedal a few times to see if it engages better - if so check the hydraulic fluid and if low top it up. Also check the slave cylinder for any leaks.

Does the clutch pedal feel spongy at all? If you can engage a gear easily, start the engine, put it in third gear and apply the handbrake. Then increase the revs and try to move forward. If the engine stalls then the clutch should be okay, if the engine continues to run then the clutch will be slipping and will need to be replaced.