As tempting as it seems to wear tennis shoes with your tux don't do it. I think it looks ridiculous. If you're 14 years old maybe give it a shot. In general don't portray anything that says 'I'm too cool and I don't care.'
I took the fear of marriage from my parents' relationship because I didn't want to end up in a relationship like that whereas my brothers and sisters learnt a lesson from it and made sure they didn't carry it on into their own marriages.
