straight from the documentation:
As you can see there, the OnTriggerEnter function accepts a collider as parameter. Basically, it tells you exactly which object entered the trigger. You can check "other" and see whether it is the player or a bullet or whatever which entered your trigger. Possibly check for the tag of your entering collider and only trigger something when the tag of your entering collider is e.g. "player"
again from the documentation:
Combine the two script examples and you should be all set
That should result in a statement like such in your OnTriggerEnter function: