I think I would probably have never taken it upon myself to start practising my family's nominal Catholicism, and instead realised much sooner that I don't need over-zealous, soul-destroying and scrupulous devotion to my cultural heritage in order to be secure in my individuality. Having said that, those years were a experience of sorts, and I suppose it was good to get the fanaticism out of my system somewhat, and it opened me up to spiritual questions, which is a great thing, I think.
I would throw away every television in the house...now it's too late...I'm a square-eyed addict. :o
I would have spent more time going out with my friends and getting to know new people.
I would get into a very early habit of working hard at school and in other stuff.
I would take up more hobbies, learn more languages, learn to play an instrument early on, go to more swimming lessons...stuff like that.
I would sneak out of the house more, and sneak back in hours after curfew more.
I would eat better and more varied foods when it was easier.
I would enjoy and appreciate everything I did much more.
There's a few specific things that, on the whole, stem from that first one...
I would have started experimenting and having fun sooner.
I would start dating sooner.
I would be totally honest about every aspect of my personality, rather than trying to bottle up my true emotions.
But, as I'm only 17 and a half, hopefully it isn't too late...:o
peace