I workout a lot and eat pretty healthy, but I just started birth control and am afraid ill gaun weight. What can I do to prevent this from happening?

Weight gain. Womens are always afraid that they will gain weight on birth control, but some women actually lose weight. Don't prejudge what will happen, continue your work outs, and just monitor it. Women always gain weight when they are pregnant., which you will be avoiding!
You should not worry. Truly, the pills should not make you gain weight. If you watch what you eat and exercise regularly, the gain of weight should be minimum or not at all. Certain medications have such tendency not because they cause weight gain, but they may cause increase hunger and people eat more, consequently the weight increase. Otherwise, the pills do not really have any caloric values!