Read through the whole play. This will familiarize you with the premise of the story, the setting and important character nuances.
Analyze the scenes and the characters involved. Look through the scenes for which you are responsible for staging the props and take note of any scenery or prop instructions and character actions.
Talk to the set designer about the background for the scene and what it will look like.
Talk to the director of the play. The director likely has certain ideas about what type of props she envisions with each scene. This valuable feedback will help to ensure everyone is on the same page in terms of the props.
Attend play rehearsals. Seeing the play performed live, even in raw form, may spark some ideas you wouldn't get from simply reading it.
Close your eyes and imagine the scene in your head. As a prop master you have a degree of creative control.
Pick props that are period-appropriate. For example, if the play takes place in the 1950s and you need a telephone, you wouldn’t want to use a touch-tone phone.
Pick props that are class-appropriate. If the characters in the play are poor, choose cracked dishes and not fine china.
Consult with others working on the show for another opinion. A play is a team effort and they’ll likely appreciate that you asked.