Just go looking at lots and lots of bikes. You'll get confused but eventually, that confusion will fade and you'll find yourself going to the same section of any shop you walk in to - THAT's when you start narrowing things down. Whatever you do, buy from your heart, not your head because if you don't love it, you won't ride it. The only caveat to that is not to spend too much because the chances are, in twelve months, you'll want something different - even if that's just a better version of the same, you'll be fitter and more flexible so the fit will be different.