I'm not sure if the OP is looking for a mathematical statistics book. Reconciling introductory statistics with real mathematical rigor is not always clear. If I'm not mistaken, many mathematical statistics texts will presume familiarity with advanced calculus or mathematical analysis. Thus, if you've never really studied statistics before (beyond really basic stuff), you might want to look at something more basic than what has been prescribed. Of course, introductory statistics books are usually not mathematically rigorous in the sense that you probably won't see many proofs. But again, I think (though I am not sure) that more rigorous treatments require an appreciable amount of mathematical background.