You can buy a mob spawner at spawn for $150,00 by right clicking on the sign.
{{ :spawner-buy.png?400 |}}
You can use them by placing them and right clicking them with a spawn egg of a mob that you catched by hitting them with an egg.
Just specific mobs are allowed to be used with spawners.

//please expand that list if possible//

====== Mobs that are not possible to be catched with eggs: ======

* Creeper
* Enderman
* Pillager

====== Mobs that can be catched with spawn eggs, but can't be used with spawners ======

Trying to set a spawner with one of those mobs will result into one mob getting spawned and the spawner 
**destroyed**. This is enforced by the NoSpawnerChange plugin.

* Shulker
* Witch
* Stray
* Cow

====== Mobs that can be used on spawners ======

* Blazer
* Skeleton
* Zombie