“Study, find all the good teachers and study with them, get involved in acting to act, not to be famous or for the money. Do plays. It’s not worth it if you are just in it for the money. You have to love it.”, Philip Seymour Hoffman.
When
Every great actor has had an acting coach, and most of the times has one to prepare for a role.
Before starting to look for an Actin coach, you have to study and learn the fundamentals of acting! You absolutely need training first.
Then start getting out there, go for castings, and once you get cast for a role, especially your first one, that is when you can consider getting an acting coach.
What
Before an audition, a coach can help you make strong choices, get over your stress and feel more confident. He/She will discuss every aspect of the uadition, from your outfit, to your hair, make-up, body language, memorization skills. A coach also helps you have another look on your scene, and takes it to a place where it feels like it’s second nature.
On stage or when filming on set, a coach will help the actor prep his lines, understand and focus on the scene.

A coach is like a trainer, he prepares the actor/actress before going into the ring.
Who
Many websites are dedicated to finding an acting coach. I still feel like the best way of doing it when you are getting started, is looking at your acting classes professors, or their recommendation. Also if you have a good agent and/or a manager they will help you find one and go to the best one to work with.

Has anyone of you ever had an acting coach and how did you find them?