If I could go back knowing what I know now, then for me, I'd go back to July 2000, to the day I collected my GCSE results, and do pretty much everything I did since then differently.
For one, I wouldn't bother going to sixth form college. I only went because I didn't know what I wanted to do with myself, and going there postponed making a decision for two more years, that and I went simply because most of my friends were going. Instead I'd get a job doing pretty much anything, and save as much money as I could until I turned 18, at which point I'd apply for a Merchant Navy engineering officer cadetship, which is what I'm doing now. If only I'd had the foresight to do this back then, then I wouldn't have got sucked into the maw of call-centre, then office work that I did, that did nothing more than make me utterly stressed out, miserable and pile on weight.
Although I'm at last headed in the right direction, I feel like I wasted what should have been the best decade of my life, and I can't help but feel bitter regret about the decisions I did (or rather didn't) make looking back.