Staying Fit - ADVICE NEEDED!

Hi all! I just found out that I am almost 5 weeks! It hasn't really set in yet that I'm growing a human inside me! AH! Anyway about 5 years ago I lost almost 90 pounds and I have this immense fear of gaining a lot of weight during this pregnancy. Does any one have any tips and how to manage a healthy weight for me an baby?! Thanks!