Lights are essential when you go out cycling during night. When out at dawn or dusk they are also advisable, as the saying goes "Better to be safe than sorry". Spend some money on a good set of lights, apart from brakes, they are probably the most important item on your bike. They keep you seen and safe. Look for the brightest lights you can afford and do not buy a cheap set, your life is worth more than that.
Locks will help your bike remain yours. Secure your bike with a decent D lock, and fill as much of the space within the "D" to make it as hard as possible for any potential thief. You need to make your bike less appealing than the bike it is parked next to. Alternatively look at getting a cheap bike off eBay for those shorter journeys.
Pumps will help to keep your tyres at the correct pressure, making cycling easier and ensuring that your tyres last as long as intended. A smaller, more portable pump is also useful for helping out in those emergency situations. It is also advisable to take a puncture repair kit with you or a spare inner tube.
Cycle Racks and bags help to make carrying items easy for you and will mean that the bike carries most of the weight rather than your back. Racks will vary from quick release versions that secure onto the seat post. These can be removed when you do not need them and since they just secure in one location they can be fitted to virtually any bike. A standard rack will secure to most bikes without disc brakes and can carry much more weight but cannot be removed that easily.
Bags for these racks vary in shape and sizes. Your bag needs will vary depending upon what you wish to carry. For the average user a small bag is adequate to commute to work with a larger bag for shopping. Alternatively look out for bags that can be expanded for when you need to carry that extra bit of luggage.
Rucksacks can also be useful however these are not recommended for longer journeys as they will make you sweat more and become uncomfortable. It is better to carry as much weight as possible on the bike.
