It's a good thing that you donate blood. After procedure, you should take a rest. Eat food rich in iron like red meat, fish, eggs, beans, black raisins, spinach etc. Also eat some sweets. Avoid any heavy or vigorous exercises like running for 1- 2 days after procedure. Take a good night sleep. Drink more water and avoid alcohol for atleast next 24 hrs. If you feel light headedness or dizziness, then lie down until you feel better.