Here is a video that help explain what is going on:
Explaining the circle of 5ths: This is very complete and in-depth. 0-3 minutes includes history and WHY? 3-8 minutes writing the major keys and key signatures. 8-11 minutes using chords to write songs. 11- 20 Transposition (moving the entire song to a new starting note) and/or Modulation (changing to a different key in the middle of a song).