I think I have some decent advice, since you asked xD. Keep in mind, I'm not someone who's accomplished great things, but I do have some ambitions that could be called great. So definitely take this with a grain of salt.

Well, I was going to ask if you knew specifically some of the "great things" you wanted to accomplish, but found out that it barely matters. First thing's first, if you don't know what you want to do that's great, find out. Let your thoughts fly, dream, and use that as a guideline to figure out just how you'll want to make your mark. This is the fun part.

The second part, which is much less fun, is you need to frame those dreams in reality. This doesn't mean settle for a less ambitious dream that's "kind of like" the one you came up with. It means taking a look at the world, in particular the parts of the world that directly relate to your dream, and see how the dream fits in the world. See what obstacles you're likely to encounter. The better your understanding of the world, the better the framework you'll be able to set for your ambitions. This framework is in large part going to tell you what you need to do. The better the framework, the clearer the path.

Here's a word of caution for you. The parts of the world you focus on will be determined by your ambition, and the depth of knowledge you seek will be determined by your definition of "great things". Depending on where you look and how deep you look, you may not like what you find.

For more specific information I'd have to know some more specifics about your ambitions. Either way, finishing school with as high a gpa as you can get seems like a good starting point for most things I can think of, seeing as you've already been on that path for so long.