You Can Be Small And Be A Firefighter by Juliet Draper

You'll never hear,"you're too small" from me. It is true that you must work extremely hard to achieve your goal. You seem to have a great attitude and that is half the battle.

You must find out what the physical ability test will be like. When you get that info you can then taylor your training more specifically.
As for general conditioning for firefighting, most tests will have you running up flights of stairs and carrying gear.

For starters find yourself several flights of stairs and start running for at least 3-4 times per week. After a few weeks put on a backpack full of books for greater resistance. gradually add time and weight until you can go for 45 mins carrying 75lbs. That is the approximate weight of full gear. Also, make sure that you can do plenty of push ups, sit ups, chin ups and run at least 3 miles. This will help you during the fire academy.

For many smaller people (men and women) upper body strength can be an issue. Free weights give much more bang for the buck. If you don't have any gym experience, check out a book called Getting Stronger by Bill Pearl. This book contains a zillion different exercises for all body parts with illustrations.

If you can, find a trainer (high school and college sport coaches work very well and are usually cheap or free.) Hands on guidance always yeilds faster results. Also, don't neglect the written and interview portions of the testing process.

Check out test prep books and look up fire service interviewing on the net. The more information you have the better off you'll be.