If you are producing your own film, you are going to want actors who have not had any screen credits. You can produce your film with amateur actors who will probably do the film for free or close to it just for the exposure.
Producing your own independent film gives you the freedom to hire who you want and not have to worry about unions or wages. The large film studios are all union shops. They have to employ union workers as far as camera operators, electricians, stage hands, etc. If you are producing your own film, you can hire who you want.
The actors in professional films usually belong to the Screen Actor's Guild which is a union for actors. Any actor who speaks a line in a film has to be credited according to the union rules. You will notice credits for actors and anyone who was associated with the film after the film is over.
One way to get the best actors for the job is to have them audition. An audition will require the actors to read lines that are in the film to see how they are able to deliver the lines. When auditioning for a film, an actor usually does a screen test. This means that they are filmed and their presence in the film is noted. The screen test is important when filming a movie because not everyone ends up looking good on film.
Some people look very nice in real life but do not photograph well. Then there are others who the camera seems to love but who might not look that good in real life. You have probably heard about actors who want to be photographed from their "best side. As no one has a perfectly symmetrical face, people tend to look better from different angles.
You want to have actors who will realistically portray the characters that you have created in your screenplay and will also look good on film. They should photograph well and be able to convince the audience that they are the actual characters in the film.
The job of the actor is to pretend that they are someone and do such a good job that they convince the audience that the character is real.
You can usually find actors who will be glad to work for a few dollars to film your movie. You are not going to get Brad Pitt in your film, but you can get an acting student. Even free acting jobs are competitive, so you will be able to take your pick of whoever you want to act in your film.
As you are doing this yourself, you will be in charge of the casting. In a large studio, there is a casting director who will be in charge of who gets to be in the film. Box office stars like Brad Pitt do not audition for roles as they are usually coveted to play anything because they are popular with the public and will guarantee the studio that they will make money.