The subjunctive is a different kind of tense because it doesn’t exactly exist in English, so- soyez courageux (be brave!)!
It is used in certain situations with specific preceding verb phrases. It shows obligation or necessity, but it can also be used with expressing doubt, and other things like emotions.
“Il faut que” is always followed by the subjunctive, for example.
“Il faut que tu fasses les devoirs” = You must do your homework! (or “It is necessary that you do your homework”)
Peruse these links for more information, and resources, and games!
About.com’s explanation and practice exercise
Clear explanation of the pattern for formation, with some important verb exceptions
Website with many great explanations, exercises, and listening as well!