This may be in there and I just missed it, but I'll add if you are still an undergraduate (or high school student) and think you might later want a PhD, take a look at a few PhD programs and see if they have any non-obvious requirements that you had best take care of before you start their PhD program.

For example, when I was a high school student applying to Caltech's undergraduate program I took a look at the graduate school section of the catalog just to see what their math and physics PhD requirements were.

They had a language requirement for a math PhD. To get a PhD you had to either be fluent in one of French, German, or Russian, or you had to have a sufficient reading ability in two of those to read the current and historical math literature in those languages.

That kind of thing is much better to take care of in high school or as an undergraduate.