If you are able to and if you get in go to one of the top schools worldwide:
London: Guildhall, LAMDA, RADA, Central, etc.
America: Julliard, NYU Tisch, Yale, etc.
If you can't afford those, there are good acting studios as well that you can go to:
William Esper Studio (NYC) (wrote 2 great books as well)
Stella Adler Studio
Terry Knickerbocker Studio
William Esper and Terry Knickerbocker trained Sam Rockwell among many others. The Esper Studio has a really good reputation just overall. Another really significant graduate of the Esper Studio (under David Newer) is John Leguizamo.
They teach the Meisner Technique. The big schools I think teach a mix of various techniques.
However, in the end, you should know that even if you go to a top school it can always happen that you don't get selected at the showcase, you're in debt and will have to hustle just like everybody else. In the end people care the most about your experience rather than your training. Training helps get your foot in the door though.
If you plan on studying in London, I
think it is wiser to stay in London since the schools are such household names there. If you think you'll study there and go back to let's say New York.. you will still stand out but I know many actors who even went to Julliard or one of the big schools in London and are now struggling to get representation just like everybody else. BUT you will stand out for that for sure.
If you can afford it, totally worth it.
I definitely say get training somewhere, do improv (UCB, The Pit, etc. in New York) and get your feet wet. Copy and imitate, listen to interviews, read books and work on your voice, diction and body. Relaxation is key.
Work with other actors who trained / have a great process and learn from them. How do you find these people? I do not know really. Book work. But that's the conundrum. You gotta have worked to book work, etc. etc.
So submit submit submit.
That being said.. there are PLENTY of actors who NEVER EVER trained and are really good. It can be and most likely is a mix of things: work ethic, talent, openness to learning, LUCK (= where preparation meets opportunity) and learning by doing. Watching others work.
"Good Artists copy. Great artists steal." - Picasso
"There is nothing new under the sun." from the Book of Ecclesiastes
=== everything is a remix. Don't feel bad about about seeing something and getting inspired by it and taking elements of it. Everybody does it.
If you're in the US, sign up for Backstage, ActorsAccess and CastingNetworks. Backstage is where a lot of stuff is casting all the time, especially smaller projects.
ActorsAccess is the place where many many big legit projects are cast as well as other smaller projects. YOU as the actor will only see the breakdowns for the smaller stuff, maybe occasionally a public breakdown for a very specific project. The agents and managers have access to the BIG LEGIT stuff.
Hope this helps. Let me know if you have questions.
EDIT:
This may help
https://www.hollywoodreporter.com/lists/25-best-drama-schools-895029