Watch Fame ( pt 1) gives you some idea what happens to young hopefuls. Find out what the drama schools want - and do that at A level. join School productions of whatever is going. join local Am-dram.
Even getting into drama school isnt easy. get in and follow the course
From my school - (500) only one has made it - Nicholas Gecks who is hearth stone in the Rumpole series
Cambridge - 1962 - Bill Oddie, Graeme Garden, Frost, Cleese - then virutally nothing for fifty years until Laurie and Fry, then Branagh and Thompson
In my year at Cge - Simon Jones ( Lord Bridie, Brideshead and Ford Prefect, Hitch-hiker)
and THAT is it....It is nice to see bit part players making it, but hell it is rare - Saul in Just Call Saul was hired for one episode in Breaking Bad ( the lolly pop bank where the Feds were waitig) and made it. Jonathan Banks too. Rami X who played Freddie Mercury has made it. Oh, Michael Caine in Zulu 1964.