Gears not engaging properly?

While starting in first gear. It seems as though that the gear does not engage properly. If I increase the accelerator then the vibration increases and the car starts moving but the engine rpm seems to go high even if I leave the accelerator. The car seems to move in little higher speed than usual at first gear. Sometimes the gear just dis-engages and comes to neutral.
Can you please let me know if it is a clutch problem or if the gear box requires oil changing or something major?
Thanks