First time i tried to have sex with my girlfriend and my penis didn't get hard. what could have caused this?

Probably GOOD: Growing up has many milestones. The fact that you did not get an erection very likely means that you were not ready to enter onto this new phase of a relationship, and/or you realized that she was not ready and you are a gentleman. In this situation -- which comes up for every guy from time to time -- be ready to use the rest of your body to make her happy, and soon nature will take its course 4u.