Qualifications
A great way to get yourself on track for a park ranger job is to start off with some kind of internship. Chances are there's a state or national park somewhere near you, and it's likely they're looking for volunteers. These positions could be anything from helping with the education programs to picking up trash, but no matter what they are, they're a great way to get a boot in the door.
You can get a part-time ranger position without a degree, but your chances aren't as good and the pay is super-low. It's best to at least get a two-year degree, and if you want to be legitimately competitive, a four-year degree is the way to go. What's two more years? Just do it; you'll be glad you did in the long run.
There are rangers out there who majored in a ton of different things. Common degrees include fun stuff like environmental studies, forestry, and biology for rangers who want to zero in on the nature-y side of things. While life sciences are the norm, you can also go for things like archaeology, history, and business administration.
If you're looking into the law enforcement side of ranging, you ought to get a degree in criminal justice, and you'll also have to complete a Seasonal Law Enforcement Training Program (source). If you have your sights set on the firefighting side of things, you'll have to take special courses (which you'll probably be grateful for when you face those raging infernos). Former police officers, military, and firefighters have a leg up on these jobs for pretty obvious reasons.