Reading through open source software can be very educational if you already have a little bit of knowledge of C++. If you're just starting out, I imagine the Boost libraries will go right over your head. You could start with the WebKit project, which is an excellent resource. Also Google releases a lot of their source code, which also happens to be VERY clean: Protocol Buffers is a great example. And while you're at it, you may as well read their C++ Style Guide as well.
Personally, I started learning C++ by picking up a couple of books and writing some little challenge applications. Bear in mind that learning a language as extensive as C++ takes a long time, like 10 years long.
Many years ago, I was told that if I ever wanted a career as a C++ developer, I should definitely read Effective C++ by Scott Meyers. That should prevent you from falling into many of the language's pitfalls. If you find you need something even simpler, start with The C Programming Language by Kernighan & Ritchie. Be patient and good luck!
