Personally I'm not a big fan of these books like Freakonomics, or the Undercover economist, if you want to read them as leisure reading then great but I wouldn't mark them down as 'must reads' because every wannabe Economics applicant reads them and writes it on their UCAS form. I bet half the Admissions tutors think these books are crap anyway.
I think for a 6th former who wants to study economics, you are better reading economists blogs online. Some of the top economists blogs will be a bit over your head because they are are basically academics blogging for the target audience of the economics community (ie other academics and postgrad level students) so their stuff presupposes knowledge. Start with stuff like Faisal Islam the Channel 4 economics editor, Stephanie Flanders blog on the BBC, Edmund Conways blog on the Daily Telegraph website, Paul Krugman does a very good blog which is accessible to the a non-economics specialist audience, various blogs on the F.T. website. If you just maybe follow two or three of these blogs and try to read them every day or so, you will get an awareness of the current economic issues and you will start to understand some general macroeconomic concepts.
If you want to have a textbook to refer to so you can read up on anything you don't understand, then I recommend buying Greg Mankiw's book on Macroeconomics, or John Sloman's big (and probably expensive) economics text. Sloman is probably the most comprehensive of the 1st year level UG economics textbooks, it doesn't go to too advanced a level but it covers a heck of a lot in there so is a really good point of reference. Mankiw and Sloman will both be really useful books when you get to uni, they will get you through 1st year.