Where do I start ?
Here. You've come to the right place. Everytime you have a question, post it - even if you think it's stupid.
When do I start lead training her ?
The sooner the better. I would start out, just getting her to follow you on a lead. If she will not move, use treats to "pull" her along.
What is the best approach ?
A well rounded one. Don't concentrate on just one thing - your dog's training should be constant and should involve both structured conformation classes (tell us where you live and then ask where you can find some good conformation classes)and training on your own. Don't forget to sieze every opportunity you can for human and dog socialization. Your dog should learn to be stable in alot of different environments - people, big dogs, little dogs, outside, inside, loud noises, etc.
What type of lead should I buy ?
Go to an All Breed show (the bigger the show the better) and find one you like - you'll have plenty to pick from. I prefer the leather braided type - they look clean and are easy to wrap up in your hand when needed. See what other bulldoggers are using and ask them where they got them.
How do I teach her to stack ?
Well, first, YOU have to learn how to stack. Then, you can practice with your dog. Best way - go to a show and ask someone to teach you. It takes less than 5 minutes to teach someone how to stack. You should consider building or buying a ramp to practice on. Lastly, once she can stack well on her own, be sure to do it in the presence of other dogs (at a training class) and in front of others. If she is reluctant to go up the ramp, prod her with a treat.
At what age sould they enter their first show ?
You can enter a fun puppy match at any age. You can start showing at 6 months. The best way for you and your dog to practice showing is to enter shows.
Your best resource is here in this forum and on the show floor - preferrably a specialty show that has only bulldogs entered. Find one in your area and go to the show and ask questions.