You have to think about your main loop, what does it do, in what order what for. The question you is basic question of a program design.
Fisrt, the event loop, that will allow you to know what user had done. You could simplify your program by using Input.
Then there is the update phase. depending on time and user input you will trying to move object and testing collision then removing them according to it.
And finally draw you object.
The best way you have to succeed, is trying failling and trying and failling.. Until you succeed. Even if you don't really know what to do, try... You also should read code from other people, start with the sfml's sample, especially the pong one. You could also read the wiki.