Ford Fiesta Clutch gearbox problems?

Basically I cannot get the car in gear when the engine is on... If i put it in gear and turn the engine on with the clutch fully depressed the car lurches forward - i can then get going and change gear when the revs are right. However it is stiff... anyone got any ideas? It is quite hard to drive so i have stopped... Thinking either clutch or one of those cylinders??? (sorry im new to all of this!!!)