You should read a C++ book before starting, otherwise this will happen all the time
Be careful, a lot of literature teaches bad practice. A good book would be the C++ Primer (not Primer Plus), or Thinking in C++ (which is a free e-book).
SFML really requires a solid understanding of the basics in C++, and it will cost you much more time when you begin directly with SFML, instead of learning C++ properly before. So, invest a few months reading only C++ theory and develop things in the console, so that you have a good fundament to start later with game development.
I know it sounds discouraging, but trust me, you will be thankful if you develop a game and don't have to lookup essential C++ concepts all the time.