Is this code from the space shooter tutorial? Just curious, as it looks familiar.
Make sure you’ve setup trigger colliders and at least 1 of the interacting game objects has a rigidbody.
You can view the trigger/collision matrix here (scroll down a bit): Unity - Manual: Introduction to collision
If this is for 2D, make sure you use OnTriggerEnter2D and the like.
Another curious question, is if this is for school, don’t they teach you how to write code there? 
If you post more code in the future, please take a moment to look at this page for how to do it nicely: Using code tags properly