Hi - good to see someone new on this thread! It's great you're taking up squash, it's a really great sport - very fun when you get used to it!
I would recommend you wear tennis-type attire, so that's sports t-shirt (maybe polo shirt) and comfortable shorts.
As for eating/drinking, usually it would tend to be the same food as normal beforehand, although I'd avoid fatty foods as far as possible and drink water and other performance fuels such as Lucozade during a squash session.
At first, you may find it a little difficult to make contact with the ball properly, as your wrist may not be accustomed to playing with a heavy racket (unless you've played other racket sports fairly frequently). Assuming you play say once a week, after about a month you should find your hand-eye-coordination improving . Technically, I suppose it'll only take about 1-1.5 months to get to a level where you can feel confident about playing competitive matches at the lower levels, but if your aim is to really master the sport, it could take a lot of concentration and commitment, as well as at least one year of solid practice!
Good luck with your squash - make sure to read up fully about the rules of the game at some point, eventually get to practise all the basic shots (forehand, backhand, serve, etc), and most of all, don't forget to enjoy the experience!