Your doubts are basically C++ doubts instead of SDL related.
I'd totally consider studying C++ and Object Oriented Programming fist before really geting into SDL, OpenGL and etc.
Don't get me wrong, but I'm saying that because I was just like you, and could only really get things on my own when I decided to learn this topics.
If you already program, it's going to be really fast to learn, and I recommend the book "Sams teach yourself c plus plus in one hour a day". Really helped me to study by myself.