One of the many questions we get is how to access service records. For Great War service records you can access these through Ancestry and Find My past but you have to subscribe to this service although it is usually available for free at most libraries.
Bear in mind that Army service records from this period were severely damaged in the blitz during World War Two and only about 30% remain, the rest being destroyed by a fire and subsequent water damage from the attempts to bring the fire under control. This doesn't include Pension records which were being stored elsewhere.
Any service records which are post-1920 can be purchased through the government website by downloading the relevant form on this page: Get a copy of military service records
There are different forms for Navy, Army or Air Force and you'll need to download the correct one.