How do I spawn a Creeper spawner?
Solution 1:
This is quite easy:
/setblock ~ ~ ~ mob_spawner 0 replace {EntityId:"Creeper"}
Of course you can replace Creeper with any other type of mob, like Pig or Zombie.
Solution 2:
Type /give (username here) minecraft:mob_spawner and then get a creeper spawn egg out of creative and right click the spawner with it then you should be good to go.