The tree pose looks simple but takes strength in the supporting leg and flexibility in the bent leg/hip. (That’s not to mention the concentration).
Try first off, balancing next to a wall so that you can place one hand on the wall for support.
Follow the steps above – get your balance on 2 legs – feel the connection through the sole of the foot with the Earth.
Focus the gaze on a spot ahead of you.
Help get your foot into position – never place the foot on the knee joint – if it won’t go this high place it onto the calf.
Try to hold the pose for 3 breaths at first – then change over and do the other side.
For information about the benefits of this pose see http://benefitof.net/benefits-of-vrksasana/