You can have education, experience, or a combo. If you want to get a better idea of whether you would really like this as a career, and get some experience, consider the VIP (Volunteers in Parks) program
http://www.nps.gov/getinvolved/volunteer.htm
Here's a typical set of requirements for a GS-5 Interpretive Ranger:
To qualify for this position, you must demonstrate that you possess either the specialized experience or the education as described below.
For the GS-05 grade level
EXPERIENCE: You must show one year of specialized experience equivalent to at least the GS-04 level in the federal service. Specialized experience is defined as experience that has equipped the applicant with the particular knowledge, skills, and abilities (KSA's) to perform successfully the duties of the position, and that is typically in or related to the work of the position to be filled. Examples of qualifying specialized experience for this position include, but are not limited to, the following: park guide or tour leader, work in a park, recreation or conservation area, environmental education, history and/or cultural resources education, and natural sciences education. Work will include independent research and presentation.
OR
EDUCATION: You must possess a 4 year course of study above high school leading to a bachelor's degree with 24 semester hours of related course work. One year of full time undergraduate study is defined as 30 semester hours or 45 quarter hours.
OR
Equivalent combinations of education and experience are qualifying.
Credit will be given for all appropriate qualifying experience regardless of whether compensation was received or whether the experience was gained in a part-time or full-time occupation. Full-time work is generally considered 40 hours per week. Part-time experience will be credited on the basis of time actually spent in appropriate activities. Applicants wishing to receive credit for such experience must indicate clearly the nature of their duties and responsibilities in each position, the starting and ending dates of employment (month/year), and the number of hours scheduled to work per week.