1. If possible, don't think about what degree you're going to get until at least your 3rd year. (This might be impossible, depending on your college -- some institutions require incoming students to declare a major promptly.) Out of dozens of students I've dealt with, I'm only aware of one case where "spend four years taking whatever courses looked interesting each semester" didn't end up with a student meeting the requirements for a degree in something -- but I've seen lots of cases where students decided what degree they wanted before starting, carefully mapped out all the courses they'd have to take, and then changed their minds later.
2. Read your textbooks before class. Not five minutes before the lecture starts -- two weeks before the semester starts. Learn the material before the instructor reaches it in class -- and then go to class and pay attention to how he/she teaches it. Aside from cases where the instructor wrote the textbook, I always found that this approach, and consequentially hearing two different perspectives on the same material, resulted in my understanding the material far better than if I only went to the lectures or only read the textbook. (Think you can't spare two weeks to read the textbooks before class? Look at it this way -- if you spend those two weeks, you'll know the material well enough that you don't need to study at all at the end of the semester.)
Seconding #1 (well, #2 too, but mostly #1). I was sure I wanted to be a physics major when I entered college. I continued being a physics major despite doing terribly in most of the upper-level classes. One of my biggest regrets about college was not switching to CS earlier, because I could've gotten much more out of my classes if I'd taken courses I was really interested in rather than taking courses I thought I had to to complete the major and then doing the stuff I was really interested in in my spare time.
Your perspective in high school is really narrow. It has to be; most of the things you might find are exciting aren't even taught in high school. (Similarly, many of the exciting parts of the working world aren't taught in college unless you got to a specialized college.) Be willing to accept all the new avenues that open up for you in college, and don't stick to your plans from high school just because they're your plans.
Number two never worked for me. Either I must have been doing something wrong, or the classes were not structured in a way which one could do this efficiently. I often found I would waste my time reading fluff content that was never pertinent. Either the teacher didn't follow the book closely, had a terrible choice of textbook, or I didn't have a context from which to read the book to put it's material into perspective.
1. If possible, don't think about what degree you're going to get until at least your 3rd year. (This might be impossible, depending on your college -- some institutions require incoming students to declare a major promptly.) Out of dozens of students I've dealt with, I'm only aware of one case where "spend four years taking whatever courses looked interesting each semester" didn't end up with a student meeting the requirements for a degree in something -- but I've seen lots of cases where students decided what degree they wanted before starting, carefully mapped out all the courses they'd have to take, and then changed their minds later.
2. Read your textbooks before class. Not five minutes before the lecture starts -- two weeks before the semester starts. Learn the material before the instructor reaches it in class -- and then go to class and pay attention to how he/she teaches it. Aside from cases where the instructor wrote the textbook, I always found that this approach, and consequentially hearing two different perspectives on the same material, resulted in my understanding the material far better than if I only went to the lectures or only read the textbook. (Think you can't spare two weeks to read the textbooks before class? Look at it this way -- if you spend those two weeks, you'll know the material well enough that you don't need to study at all at the end of the semester.)