I'm going to call my girlfriend's dad when I think she is going to kill herself
I'm not sure what reason you have for calling her father instead of helping your own girlfriend yourself?
I don't understand why she would want to end her life. Our relationship is not that strong, but I want to help her get through this so she doesn't leave me, but I can't figure out what's wrong with her.
Whenever you fear that someone is thinking about ending their life, you should take action.  If you can't help her yourself, find someone who will.
I will ask around to see if anyone knows what's wrong with her. I'll try to be there for her.
That is a really good attitude to have about it, and you must really care for her if you're willing to go to such lengths. You should try talking her into going to a therapist instead of speaking with her father, because I feel like therapy may very much help her with her depression and suicidal issues.