My suggestion is to get a really good layman's grounding in classical physics and quantum mechanics first. There are a LOT of good popsci books out there on general relativity and quantum mechanics. I would read at least 3 or 4 of these first till you have a good understanding of what came before. And what's still used 99.99% of the time. String theory and its alternatives are exciting and ARE grounded in solid math but they are not grounded by evidence or the scientific method, and are for the moment largely immune to proof or disproof. This puts these theories in a dangerous gray limbo area where they start to take on aspects of philosophy (not that there is anything wrong with that
😉 ) Again I'm not saying they are psudo science they are absolutely NOT but without any viable way to subject them to testing at this point they aren't well enough grounded as they should be. Anyway caveat aside once you have read those or if you already have a good popsci understanding of the fundamentals then I'd recommend Brian Greene's The Elegant Universe:
http://en.wikipedia.org/wiki/The_Elegant_Universe