Nik,
Welcome. Start here:
https://airlinepilot.life/t/what-does-an-atp-student-need-to-do-to-be-successful/10421/5
We highly recommend taking as many written tests as you can before you start.
https://atpflightschool.com/faqs/acpp-prep-written-knowledge-tests.html
There’s lots of information on this forum. Keep searching and you’re bound to find more helpful information. We’ve tried to make the information easy to find. Let us know if you need help.
Tory