Assuming you are after UK records, then perhaps a word of warning should be in order first. Millions of men were mobilised at one point or another during WW1, but only a small fraction of those records survive.
The reason unfortunately is down to a certain war some twenty-odd years later, when a certain Mr Adolf Hitler launched the Blitz against London. Sometime around 1940 a bomb hit the MOD building where all the WW1 records were held, and the building went up in flames. What files weren't destroyed by the flames were waterlogged by the firemens hoses trying to put the blaze out. Estimates vary of the scale of destruction, but is generally assumed that the odds of finding a surviving record are somewhat less than half. Perhaps as many as 60% of service records from the era are simply no longer with us.
Those that are are at the National Archives in Kew, London, split into series - WO363 and WO364. Series 364 were reconstructed in the 1940s from pension records mostly of those who survived the war and were wounded. Series 363 contains what survives of the 'burnt' documents. Some soldiers have just one or two pages, some nine or ten. It's a lottery, and it isn't especially based on a man''s surname or anything. A mans surname who begins with "A" is no more or less likely to have survived than a man whose name begins with "Z".
Ancestry.co.uk have started putting these records online now, so if you can't get to Kew in London, you can buy some credits and view what survives online - bearing in mind that it is quite likely you will be one of the unlucky ones, in which case your only recourse is to try and find him in the medal rolls (series WO329 I think from memory) which are online at the National Archives website for about £3.50 a go - not that they are designed for use by the average layman - like most military forms, they are heavily abbreviated. The part showing the place of a soldiers entry into the war is usually simply F&F (France & Flanders), and it is really just a list of the medals each man was entitled to the 1914/15 star, etc. Most regiments and battalions kept war diaries, but although these might mention officers by name, probably won't mention the rank-and-file. It will give you some idea of where a unit was on any given day though, though in the latter stages of the war inparticular misreporting of men and units was quite common as understrength battalions were merged into others. My gt grandfather was in the 10thBn RDF (Royal Dublin Fusiliers) and died in March 1918, but the 10thBn had actually been disbanded by then, so I'm not quite sure which unit he was with when he died, so the details on his death certificate are erroneous. Given the general chaos of the time, I can't say I'm surprised. Finding WW1 records is just not that easy.