If you want to look into the military service record of a soldier who served in the First World War, there are three sources which are invaluable in giving you the background information to make a start.
1. Army Service Records of the First World War by William Spencer (Public Record Office).
2. Tracing Your First World War Ancestor by Simon Fowler
3. Tracing Your Army Family History, - an Imperial War Museum guide.
If you know that the soldier lost his life while on active service, there are two prime computer based resources from which to start your search;-
1. The Commonwealth War Graves Commission website
and
2. Soldiers Died in the Great War , - an Ancestry.co.uk website.
Other vital military records available on Ancestry.co.uk and other genealogy websites, include;-
1 The Medal Rolls Index, - a card for each individual soldier where service medals have been claimed / awarded.
2 Service Records, - only about 30% of these survive, as a consequence of enemy action in the Second World War.
3 Pension Records, - if the soldier was discharged to pension. Again, only a limited number of these records have survived.
4 Silver War Badge Records, - if the soldier was discharged during the war due to wounds / illness, these badges were issued so that the soldier could show publicly that he had "done his bit".
Once you have started your research, you may want to establish further details.
The links below are for additional resources to help you further your research.
The Green Howards Museum
A Family Memorial, - Medals and Scrolls