Nearly 6 million men served with the forces during the Great war, therefore the chances of you not having a family member in the war is fairly remote. So now you want to find out more about that relative of yours what information do you need. The simple answer is as much as you can get, and it comes in all sorts of forms.
Your aging relatives are a tangible link with the last, and may well have a good few stories to tell,ask them they can only say no, however beware they may be confused about events that happened 90 plus years ago, and whilst they were still a child or not even born.
A good source of information particulary if they are named on the back, from personal experience I can tell you that they very rarely are.
Birth / Death / Marriage Certficates
If a man was serving during WW1 and one of these events happened, there is a fair chance that his civil profession and military standing will both be indicated ie : John Smith miner/ Pte 12345 8th Battalion Royal Scots.
These are a good start as they will give you the person name and service number, the service number is particulary useful if your relative has a common name, for example I was asked to identify a James Smith who had served with Gordon Highlanders, there were literally hundreds who had served and this did not include J Smith's , so service numbers are very useful. Armed with these facts you can access your man's medal index card from the National Archives or Ancestry which will hopefully reveal more information.
Here is and example of a card.
This particular card belong to Horace Macaulay, a young officer from Eskbank, Dalkeith. If you look at it , it may seem a bit confusing at first however I will take you through it. Name , self explanatory, Corps the unit that the man served in, Horace was a Lance Sergeant 2311 (Corporal) with the H.A.C. The Honourable Artillery Company,he was the Commisioned into the 3rd Seaforth Highlanders as a 2nd Lieutenant then promoted to Lieutenant with the 7th Seaforth Highlanders. It also states that he served in France and arrived there on 26th July,1916, he served with them until his death on 25th April,1918.
So where can I find these cards then?
Whilst these are undoubtably the best source of information, the chances of you finding the man concerned are limited, about 1 in 3 to 1in 4. This is because the vast majority were destroyed by fire in the Blitz. Even the ones that remain are in varying degrees of readabilty due to burning / water damage. These are stored on microfilm at the National Archives in Kew,London. They are also available up to the letter P again on a pay perview basis on the Ancestry website
Here is an example of a page from a military record
What about the regimental museum, my Grandad was in the Royal Scots?
A number of things to bear in mind about regimental museums, they do not hold service record, with the exception of the Guards. They tend to be run by volunteers who although generous in donating their time are exceptionally busy people. Access may be a problem as well and an appointment is sometimes neccesary. I would think about going to the regimental museum once you had exhausted other avenues.
The local newspapers of the time.
A goldmine of information on local lads, some even run to photos and extensive stories.These can normally be accessed at your local studies library.