Beginner: Tree pose is a great place for a beginner to improve their asymmetrical standing balance. For now, place your raised foot against the inside of your grounding leg wherever you feel most stable. You may keep your arms in prayer position until you feel strong enough to send the arms up through center on the exhale breath.
Advanced: Tree is a comfortable pose for the advanced yogi. Focus on pulling the kneecap up the standing leg. Check the alignment of the knees and hips. It is common to turn the knee out too far. Are your hips both facing forward? If not, you may be turning the raised leg out too far.