What Causes Pain In My Legs After Waking Up?

hi Doctor...In the mornings when i awake and put my feet on the ground it pains terribly...for some time after walking around then wares off...again after a long rest(like sitting on a chair or long drive) the moment i put my feet on the ground i am unable to walk properly for some time...then gradually it wares of...can you please help me...what could be the reason...Thanx