Military service records and other information about their regiment are an invaluable resource because they provide information such as including dates of enlistment, the units in which they served, ranks held and personal details such as physical descriptions, medical condition, dates and places of birth, next of kin and medals and awards. In order to know where to begin your research, look for any diaries or existing family documents, photographs of your ancestor in a uniform, medals or other heirlooms which might give you a clue. Local newspaper reports as well as family stories and artefacts are also a good starting point. Oral traditions passed down from one generation to another may provide additional clues on whereabouts your military ancestor may have served.
Military records are often completed a they use abbreviations and terminology which, if not understood, can be a challenge. You will become familiar with these as your research progresses but there are lists available both online and from the authorities of the commonly used abbreviations.
If you are lucky enough to have older relations who may have first-hand information or may even have served alongside them, you should arrange to talk to them before it is too late. In reality you may be lucky to still have someone alive who served in World War Two but there is no living person who would have served in the Great War, although there may be diaries and letters home which may help in your search of official records.