When you start to feel light headed take the weight off your feet and lay down or put your feet up and rest.
Lying down will encourage the blood to flow to your brain and after a few minutes you should start to feel less dizzy.
Then begin to slowly stand back up but sit back down if you start feeling dizzy or lightheaded again.
Stay hydrated and avoid getting overheated and take deep and steady breaths if you feel lightheaded.
Also make sure you're eating and eating healthy meals. Being hungry and not eating can cause you to get weak and dizzy as well as lightheaded.