My joints are little week and I do yoga postures, how can I take care of them with food...?

Healthy diet. Muscles get stronger when you stress them with a load. Make sure your yoga focuses on strengthening poses and not just flexibility. Gradually increase the difficulty or length you hold certain postures and your muscles will respond. Make certain you are getting adequate protein in your diet: eggs, dairy, nuts, meat, legumes, yogurt, peanut butter, etc. Building strength takes time & consistency.