I get erections when talking with my dad about my penis...normal?

Dude its normal only if you're very young like your mid teens. Its nothing to worry about, however if you're not in your teens, i can't help you there. Darn, I didn't notice that part where you said you were 15, so in this case its normal. I used to get erections at that age for no reason at all, when i'm calling in front of class to write something on the blackboard, i would get one, i grew out of it by the time i was 17.