Yoga and sannyasa are stages in the same spiritual path. Yoga is when a bhogi, materialist, has just turned inward and is seeking the higher. He still has worldly desires but has understood the futility of chasing after them. Sannyasa is the advanced state when a spiritual aspirant has gone through yoga and has divested the personality of desires.
The first step to becoming a yogi is to renounce sankalpa. Sankalpa is an idea, fancy, wish, plan, thought flow that results in desire.
Why renounce sankalpa?
1. Desire comes in the way of object of desire. As long as you are craving for a thing you will not get it.
2. As long as your happiness rests on a future acquisition you will never be happy. The present will always be unsatisfactory.
3. The moment a desire is fulfilled many more desires spring up in its place.
The idea of renouncing desire is distasteful. But you have done this all your life. When you grew out of infancy into childhood all desires of babyhood vanished. When you experienced the thrill of teenage life childhood desires were forgotten. So renunciation is not giving up things. It is the understanding of higher joys, compared to which present preoccupations fade into insignificance.
So the first step to spiritual life is knowledge of the higher. Then automatically desires drop and you become a sannyasi. You are then left with only a few desires to gain Enlightenment, serve the guru and work for the wellbeing of society.