Welcome to the forum, the others will chime in as the day continues. You did not mention if you have already taken an introductory flight or flight experience at your age, but let me give you my insight.
ATP advertises right on their website how much they pay their flight instructors: see link. Flying as a Certificated Flight Instructor (CFI) / ATP Flight School
This may be an @Addison question, but I came into the program with 6/7 writtens completed, every written I took I handed in a receipt, the test results and ATP reimbursed me from my program cost the funds it was for me to complete the writtens outside of ATP.
This question gets asked frequently, if your goal is to instruct for ATP, depending on your location of choice…sometimes it can be a little wait. You’ll find that at popular spots like Florida, California, and Georgia will have longer waitlists (I’m sure there are other locations that I forgot with waitlists, I can’t see them). The day you start your program is really day 1 of your interview with ATP, if you perform to a high standard, be professional, and carrying yourself with success (minimal checkride busts) the opportunity to instruct for ATP will come.
How hard are you willing to work? Depending on your work ethic it could take a year and half, to three years. Granted student load is another topic, ATP ensures that their is a fair distribution between instructors to students. If you instruct at ATP don’t be surprised if you have one student for a month or two, there is some transitioning time and this is the most predominant time for mistakes to occur, so you’ll learn very fast. By the time you enroll into ATP and finish your training program, everything should pick up and you’ll be busier than you ever could imagine…granted I don’t have a magic 8-ball and can’t predict the future.
Continue working hard in school, accomplishing good grades, and take a look at an introductory flight to make sure that you are going to be comfortable in a small plane. It’s quite a different feeling than sitting in the back of an Airbus or Boeing going cross country.
Please let us know if you have any other questions, this forum contains a ton of question and answers, as you’ve already seen.