With needle phobia, people will often faint at the sight of blood, but it usually requires the sight of blood, or the expectation of an injury or needle that might produce it. Everybody's blood pressure drops a little at the sight of blood - it might be yours, in which case lowered blood pressure is good - and people with needle phobia just have too much of a good thing. In that case, it's usually helpful to do things to raise your blood pressure, i.e., flexing and tensing your large muscles.
But if you experience light headed sensations in non needle situations, it might be a more generic anxiety which is not associated with fainting, just feeling faint. Short, shallow breathing often creates these sensations. A good belly breathing exercise can be a big help in this situation, but you have to learn to start with a gentle exhale, and not gulp your air. For instructions, take a look at this article:
Anxiety Coach
Hope that helps.