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Depends on the Type of Ranger, Federal has two Types, Peace Officer and Interpretive. State Usually just has one, which is a Peace Officer.
A Peace Officer can be anyone with a clean record who enrolls and passes the Cadet Academy.
The Interpretive Ranger is usually a seasonal employee and often called a Park Aide as a position title. They are usually very very highly in demand and anyone with interest, a clean record and a decent work history (or none if you are really determined and have a clean history with good work ethic)
The biggest and easiest way to enter the Park System is to search for Seasonal Jobs using your States Job Search Site, Like California is called CalCareers, Federal is USAjobs, Search for Park Aide, or Seasonal with Department of Parks and Recreation, or Dept. of the Interior.
Hope this Helps!
Here a Pic when I worked Seasonally at the California State Parks, some amazing places and amazing time and you wont regret it. Pay is pretty shitty but the job experience is worth it.