Replying to Summer Park Service Job (Colorado)

So I've been wanting to get a summer job in the mountains for a while but never really wanted to quit my other job as a lifeguard. This summer I want to change that. I want to find a job at one of the many parks Colorado has to offer. But I'm not sure how to get my "foot in the door".

Are there any requirements for joining the park service?
Any of you Coloradans know of good parks to apply to?
What is it like? boring? laborious? fun?

Any advice appreciated! thanks in advance!