Basically, to make a start as an actor, you'd need to -
1) Have loads of natural talent.
2) Get a few years of training at a good, accredited acting school, going from the basic classes to advanced ones.
3) Get experience in a wide range of stage plays and shows – not regular high school stuff
4) Learn to sing and dance if possible.
5) Train for a good ordinary job as well, one you can work free-lance, as you’ll need it. Almost all actors work at least one other job just to survive.
6) Ideally study Drama at degree-level at drama school or university. (In the UK it has to be a top drama school)
7) After all the above, try to find an agent who will accept you.
8) When you get an agent, which is how you find auditions, try to win a few auditions out of the hundreds you'll have to apply for.
Just bear in mind that all that hard work isn't enough - you also need huge talent, dedication, determination, resilience and luck.
And sadly, the best that almost all actors will ever get is about ten days’ paid acting work a year - that's the average - and will never get more than a few minor roles in small productions.
Watch any movie, and count the number of actors in the cast list at the end which you hardly noticed - 'fifth bad guy', 'blonde prostitute', 'cop with ladder' and so on - they all imagined themselves up there at the top, but for the vast majority, that's as good as it gets.