no need to buy a flight bag just yet, I'm still using an old laptop bag which has plenty of space to fit all my stuff in it. during the first part of your training all you really need is your logbook, kneeboard, map and a couple of pens.
I agree with 28thJuly about turning up with a bag crammed full of all but the kitchen sink. I was bought a flight bag for christmas but it still stays at home as I'm still finishing off my circuits and only need a few bits and pieces.
I did consider using a Sainsbury's bag at first, but I still find my nutri-grain bars get squashed by my book.