If someone has a potential to become an actor, what’s the best way to start his/her journey?

Alan Chenkin, Learned about celebrities by reading the tabloids in the checkout aisle.

We all have potential. Even if you feel you don’t have any, acting skills will help you find your way.  If your potential and desire is in the acting field, Learn to ACT!
Take classes. Audition for everything. Learn passages from works that you can memorize and use (at a moments notice) for impromptu auditions.  Stand up and perform at a moments notice – lose your fear of public speaking,  thrill your friends with impersonations, spontaneous displays of a “character” you are working on, and OWN IT.
Build an acting resume.  get a good black and white headshot, and use those as your calling cards.
Develop a thick skin. Learn to value criticism, and use it to improve your abilities.  Practice will often trump talent.  No one is “born” an actor – the skill must be practiced and rehearsed until you are a “natural”.
Find mentors and seek out an agent.  Acting jobs, fame, and fortune may not seek you out – you have to go forth, with all the acting chops you can muster, to find your breakout roles!
Break a leg!
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Expanded from my answer to this question on Quora.