First and foremost get yourself a helmet, to be honest they can range from £25 to £225 and only real difference is the weight which again only makes a difference in competitive cycling. If you are getting a hybrid to knock about on and do some medium length cycling then anything up to about £50 would be fine. I just bought this one http://www.evanscycl...helmet-ec025853
I have no lights on my road bike as I don't intend to cycle it during twilight hours but I do on my hybrid as I will use it for training on during the winter and occassionally at nights. Lights if I was to be honest do not really light the way that well and I just think it is too dangerous cycling on the roads at night.
Shorts are always a good idea and can also be expensive. Depends again what you are doing. I prefer lycra as it shows off my rippling muscles but a load of the guys I know use baggy type shorts still with arse padding which is pretty important.
I would also no be without my clip in pedals and cycling shoes but these could probably come later for you. You will probably be provided with pedals with straps on them and any old trainers would do until you feel the need for more speed!!
Other more sensible folk would also recommend buying spare inner tubes, tools, bum bags, pumps and all that jazzle but again not 100% necessary and something you can ask for for birthdays and xmas etc.