In the morning we set out for Crater Lake, thinking we might two days there and celebrate the 4th of July at the much written about sight. We have been to enough national parks to know that you really need to dig in and learn about the park, in particular its' hikes, special features and experiences that you may need to sign up for in advance or early in the day. Doing research before you arrive helps you begin to understand the park before you get there and often gives you insight into special exhibits, happenings and closures (such as roadwork or trails that have been closed due to construction, weather damage, fire, etc.).
That said, once in the park there is so much more available to help you understand the park like going to the visitor center (often there are several in a big park), watching the informative movies about park history, geology and wildlife, really digesting the newspaper or brochure they give you about what is going on that week (ranger led talks, hikes and programs) and beginning to absorb the map of the park and the timing of getting from place to place.
Only after you have done these things can you take full advantage of the last, best resource; people. The rangers themselves, the interpreters who work at the park, and other visitors can provide a wealth of knowledge, but not if you don't have a cursory understanding yourself. Knowing what you like and what type of experiences you are looking for and having a general understanding of where things are and how much time you have will help you to comprehend when they refer to something and explain how to get to it.
The reason I am going into such detail on this is that we did not do our research on Crater Lake. It is by no means one of the "big" parks so we thought it would be easy to figure it out when we got there. I think a little research up front would have turned up something about the time of year we were visiting the park. As you will see, it would have been helpful if we knew what we were in for.