How do I make powerups? (bullet damage)

I'm trying to make it so that once I get a powerup, my bullets do more damage to enemies. I'm not sure exactly how to do that. If I need to use global variables, how would I go about doing that? Any help would be appreciated.

There are several ways to do it.

Probably the best would be adding "damage" instance variable to bullet object. When the bullet is fired (spawned) set a value to this variable. For example, without the powerup your bullet.damage=10, with powerup bullet.damage=20

When the bullet hits an enemy, substract bullet.damage from enemy's health.

As for the powerup itself, this also can be an instance variable on your character object, or a global variable.