I went up thinking that I probably wouldn't go to Hall much, and would stick to cooking my own food. This didn't turn out to be the case. In the long run, I'd generally make/buy my breakfast and lunch but go to Hall for dinner in the evening. It's really cheap, and they do lovely puddings!!! (custard is particularly good). KFC is another annoying dent in your bank balance, but Hall food, and going to Formal is VERY cheap, so I'd say make the most of it!
About not drinking - I'm not much of a drinker, and in Michaelmas term especially I hardly had any alcohol, just the odd glass of wine at events and formal etc. This did NOT stop me having a good time. Don't worry! Just as I found at 6th form college, the majority of my friends drink and I fit right in regardless of the fact I'm sober. (ok, so not something one should readily admit...) And in fact, I was still down the bar every night with the rest of them, only with the diet coke. In freshers' week, if you're really wanting to avoid the alcohol, they put on a non-drinkers option on every night too. Id still say go on the bar crawl though, even if you're not drinking; I got to watch all the debauchery and make loads of new friends through telling all the stories afterwards!