For the most part, this game is fairly linear. There will be places that you can't get to. This usually involves solving a puzzle. Sometimes it's easier to shoot a portal through a door. Sometimes you have to do it the hard way. The best I can tell you is to keep at it. Really, you shouldn't have to read this guide for every level. The game is fairly straightforward. The only level I had any problem with was level 18 (They didn't fully explain the concept you need to use in the last room of this level, so I was stuck). But now, I am thinking with portals!
Something else to note is that you can't place a portal on moving concrete. When concrete starts to move, any portal that was on it will disappear. Keep this in mind, as some of the Advanced Rooms exploit this fact.