Focus on getting through undergrad. Undergrad has VERY little impact on grad school--other than VERY basic fundamentals, a work ethic, and, of course, getting in. Once your in, you should spend your masters figuring out what you like to do--because graduate work (and being a professor) involves research. Particularly if you want to be a professor at a major university--most DEMAND that you bring in a certain amount in research grants and/or fund graduate students.
I'm currently in my last (hopefully) year of my PhD in engineering mechanics. I plan to go onto academia--because I truly enjoy teaching AND I like doing research. But I have a LOT of friends who said "I want to get a PhD" that have fallen by the wayside--never because of talent or ability, but more because 1) they found something else they wanted to do or 2) by the end of their masters degree, they couldn't deal with the horror of being a graduate student (because, trust me, if you have ever felt unappreciated and underpaid, get a bad advisor for your masters or PhD.... it will definitely give you a new perspective).
But anyhow... work on finishing that undergrad. Do a little undergrad research, if possible. Explore your options. A PhD in math usually encompasses a two year masters and then a three to four year PhD.... its a LONG commitment (some people go directly to a five year PhD program... but I hesitate to recommend it, because if you decide you want out, you don't get a degree, whereas you can quit after a masters).