Which arm is preferred for blood donation , right or left ??

Does not matter: It is your choice. If one arm has a more readily accessible vein, that would be preferable for ease of collection. I commend you for wanting to donate blood. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, low fat milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Practice safe sex.