Congratulations, you're having kittens!
Exercise is good for both you and the baby, so good job! Obviously, avoid contact sports, and activities such as soccer, basketball, hockey, horseback riding, and vigorous racquet sports. Other than that, a lot of it depends on how you feel.
You can pretty much exercise as normal unless it causes you discomfort related to the pregnancy, or unless you experience complications. And don't lay on your stomach after the first trimester, don't hold your breath, and avoid isometrics. Stop exercising immediately if you experience vaginal bleeding, trouble breathing before exercise, dizziness, headache, chest pain, muscle weakness, calf pain or swelling, or decreased fetal movement. Or your water breaks or you go into labor.
Hope this helps, good luck!
Joe