Some people take everything with them - waterproofs, spare tube, tools, pump, something to drink, etc. Some people like to dress the part - cycling shoes, lycra gear, padded underpants, etc. Some people like to add accessories to their bikes - lights, GPS, lock, sprung seat, pucture-proof tyres, alarm, bell, comfort grips, pedal clips, mudguards, etc. What you think you need is up to you. I would say the only essential thing is a puncture kit and pump unless you have pucture-proof tyres.
I ride in normal clothes and shoes, and I have puncture proof tyres, and I don't like to load my bike up with unnecesary junk, though, because I use my bike for shopping and other utility stuff, I always have to carry a lock. The only other accessory I think is necessary is a bell of the bring-bring type, which you can buy from Halfords for about £7.