Go into settings -> applications and then find your current music player, open it and select "clear defaults". Now go to open google music and it should prompt you to choose whether or not it will be your new default music player
Try this: Delete google music, and then re-install it. Go to "My files" and locate a song and open it. At that point you should be prompted with which application to open it with and to also set that application as default, choose google music at that point.