For your own safety, it is important to do regular check-ups of your bicycle. Especially if you are using it all year long as a form of transportation. Breaks, gears, chain, bell, reflectors, and lights are the most important bike parts to look after since they are keeping you safe. Also, make sure you have a good helmet even if you only are riding on bicycle roads. If you are just starting to bike ride you might want to read up on traffic rules for cyclists.
If you have bought a second-hand bike you might need to change quite a few parts, which can be expensive. But if you buy the parts from the compare site bike-spare-parts.co.uk you'll get a better deal than you would from a bike store or from other sites on the internet. This site collects everything from wheels to saddles from many different sites so you can make price compares and buy your products as cheap as possible.