Unfortunately, service records from World War Two are not yet in the public domain. If you are researching a WW2 man, you will need to contact the Ministry of Defence. The procedure is straightforward and is clearly outlined on this MoD website.
I'd love to be able to add The Second World War to my research topics but as I say, service records form this period are still with the MoD and so my advice for World War Two research is always to start there first. But if you need help with pre WW2 research, please visit my research page HERE.
To find your pre WW2 army ancestor, try searching on Ancestry, which has recently updated the way it presents its military records.