There are a number of different ideas and thoughts about quarantine. If you are buying all your fish from the same aquatic shop, then you are likely to already be sharing their particular "micro ecology" of specific bacterial elements and microscopic fauna. This is not something to worry about, in fact, as long as you are buying healthy, hardy fish from that same single source, you can normally add them without need for quarantine.
If however, you buy from multiple sources (online, other retailers, or fish from other hobbyists' tanks) then quarantine can be a very useful and efficient tool to ensure that your aquarium remains healthy. If you intend to keep more delicate and disease-prone species such as Discus or certain species of marine fish (especially if they are to be added to a reef tank where medication is challenging), then a quarantine period might be advisable, even if they are from your regular store.
If you have had a tank established without addition of livestock for an extended period of time (years or more), then it is definitely a good idea to quarantine any new animals. Your tank will have been isolated from many pathogens and your pets' immunity will have been shaped by the tank conditions. The addition of new fish brings new microbial challenges and your native initial population could suffer.
There are many different methods of quarantine and to list them all would take considerable time. The basics, however, are to isolate the new fish in a different tank to your main display BUT sharing the same water parameters. To do this, you need an appropriately-sized tank that is adequate for the needs of the new fish and a matured filter (or mature media that can be placed into a filter) suitable for the quarantine tank. Temperatures should be identical to the main tank or possibly slightly higher to boost the immune system of the fish and speed up the life cycle of any parasites the new stock may be carrying. Provide shelter and hiding places if needed to ensure the comfort of any inhabitants and minimise any stress that can lower fishes' immune systems.
Fill the tank with as much mature water as you can spare from your main tank. Acclimatise the newly purchased fish as you normally would, and then leave the fish in the quarantine tank for a period of 2 to 4 weeks. Feed as normal and perform water changes weekly. After the time has elapsed and assuming the fish have shown no signs of illness or disease, they can be introduced to your main tank.
Whilst in quarantine, some fishkeepers will treat their fish with medications or treatments to kill bacteria or parasites. While preventative treatment can be a good idea, always remember that any treatment given to an aquatic animal can be stressful if not properly or appropriately administered. Your local Maidenhead aquatics store can advise you on what is best on a case-by-case basis.