Most take it the summer before their senior or junior year.
You are young so here is some crucial advice (at least I think so) for your freshman year. When you take general biology and general chemistry, do all the problems at the end of each chapter in the text book, even if they aren't assigned. If you don't understand one of the problems then go to your professor. They will be very impressed that you are doing extra work, which lets the professors know you better so that in the future they can write you a nice letter of recommendation!