How to become a herpetologist?

Sounds like you've got the schooling mapped out and understand that you need to study hard and such, so I recommend also getting experience working with and handling animals. Try and get a job or work as a volunteer at a reptile sanctuary or a zoo or marine park. And don't worry if you can't work with the animals that interest you most right away; venomous reptiles, sharks, and big cats take a lot of training to work with and you're only 14 at the moment, so there may be rules against allowing you to work with those particular animals. However, working with animals in general is going to help, and you may be able to care for nonvenomous reptiles and other fish and such. Also, you can talk to the people that work in those places about ways to prepare for your career plan.