I didn't buy food in town until week 7 or 8, although I was supplied with large amounts of food when I went up, and after my birthday a couple of weeks in. If you want cheap food, there's a place in town called home bargains, which is great, and has really cheap food, as well as stuff like shampoo and other things.
Societies are good for social stuff, they can have some slightly wacky, but good fun things going on there
Central tends to be a little bit cheaper on things that are there and in Spar, as well as a slightly larger range of weird things I'd never seen before, but taste great.
There are lots of bars on campus, which can be fun, each with their own "feel" and slightly different drink menus. You aren't forced to only drink in your colleges' bar