Calf not knees indeed, but you say you haven't had many lessons. I would advise you have a neck strap or martingale so you don't try and balance yourself with your hands.
Also, I know its great every now and again, however its not too great for the female anatomy, hip wise. Someone once told me what it was, but I can't for the life of me remember what it was.
It will take time to build up the calf muscles for rising without stirrups, and lots of practise. Just try and sit tall and use those legs ;)