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).
First of all take off your shoes – it’s much easier to balance when you can feel the soles of your feet on the floor. Chose a room where the floor is stable and level. Never try to balance on a rug or slippery surface.
Try balancing next to a wall so that you can place one hand on the wall for support.
Lift one foot off the floor and turn out your knee. Be gentle, place the sole of the lifted foot next to the standing leg – low down to begin with.
Focus the gaze on a spot ahead of you. Breathe a few times to see if you can hold the pose still.
Next try the other leg.
When you feel confident, move away from the wall and try the hand position.
As your balance and flexibility improves, you can move the foot up to press onto the inner thigh. Never press the foot onto the knee joint.
For information about the benefits of this pose see http://benefitof.net/benefits-of-vrksasana/