You will know what God wants from you as you start purification. Only a pure person knows what God wants from him or her.  According to the Gita, purity comes by  Yagya, Tap and Dan -- selfless service, controlling your desires and helping needy people. These are the basic meanings of purification: Yagya, Tap and Dan. Without that there is no point in even hoping that you will ever know God, what he wants or what you should be doing with your life. 

This is a very appropriate question. Do not take it lightly. You aren't going to appreciate or grasp this sometimes because it is beyond your intellect and beyond what you have been taught. You need a new dictionary, a new language and a new way of life to get this questions answered. Only spirituality and the Gita has the answers to such questions. If you are given simple intellectual answers, you will not be satisfied. Maybe your brain will, but your brain is a poor instrument and it is going to give you problems just like the mind.

Try to satisfy your soul rather than your body, mind and intellect. They can be satisfied when the soul is satisfied. At the end, when you grow old and if the soul is not satisfied, you will wonder what happened to your life -- you will realize that you are a part of God and that you have wasted your life in vain.

--Swami Radhanandaji