My husband and I decided to start trying to concieve when I run out of birth control in 2 months. When should I start prenatals?

Now. You should start prenatals when you are trying to concieve.
Now... but. The cited average wait for a desired conception is 6 months with a wide range from weeks to a year. Beyond a year a review of fertility issues becomes valid. Be patient, things should go well & taking prenatal vitamins will help prepare your system for when it happens.