The National Archives, located by the side of the Thames at Kew near London, is the repository for a vast number of records, and is the main archive for military records.
It has its own excellent website and on-line catalogue. As well as giving useful information about the archive, the website has a large number of helpful leaflets which can be downloaded, and these will tell you where to start your research if you are thinking of visiting the archive. They cover a large number of subjects, from Admiralty Charts to World War 2, and I highly recommend that you have a quick look to see what is available in your area of interest.
The on-line catalogue that is available over the Internet has exactly the same appearance as the one that readers use at Kew itself. You can use it to browse over the various categories of record, or you can search for a particular name, perhaps of an officer or of a ship. The on-line catalogue is supplemented by a hundreds of printed indexes, and it is often easier to look for something in these printed volumes.
The building is very modern, and the document ordering process is highly automated. The progress of the material is tracked at every stage of its journey by using bar code scanners, and service is friendly, efficient and knowledgeable.