Whenever I do asanas that involve me putting body weight over my hands, my wrists start paining and i am unable to sustain the posture for long. What should I do?

As you progress in the journey of yoga you will gain the strength to carry your body weight. In the meantime, please do as much as you comfortably can and take a break and rest whenever there is pain. Remember, it is a journey and slowly and gradually you will see improvements.