Get as much experience as you can. Take various drama classes, perform in plays, get experience from multiple aspects of theatre (improv, acting, screenwriting) perhaps as it will show that you are familiar with more than just the acting component in teater. Build an acting resumé that includes all your best work, get headshots, make sure that you have recordings of some of your best performances (for applicatons/auditions in the future). Apply to a good drama school; prepare for this application far in advance so that you can have the best application possible. Perhaps get an agent. Once you have done all this there are of course different steps and things you can do, but what mentioned here is pretty much how you "become an actor" the traditional way.
It all really depends on what kind of acting you wanna do, and if you want it as a hobby vs career. But these are the tips I would always hear all my friends who are professional actors give out to people. Good luck!