Hi Valerie
I assume from your request that your grandfather survived the war in which case try the following. (It worked for me).
Go to
www.nationalarchives.gov.uk - then select "Military History" - then select "Search the Archives" - you will see the "Documents Online" section. Click on the "GO" box and this will bring up the WWI Campaign Medals section - click on "GO" and you will be presented with a selection of boxes where you can fill in the known details of your grandfather - then click on "search".
This should present you with his medal roll which will display all his medal entitlements plus a bit of info on his rank and where he served.
The only problem is this card does not include his full service record as very few of them survived destruction during the 2nd WW. You will need to go to Kew to search manually for the few that have survived ( more info on the national archives website).
Anyway, good luck and hope this helps.
Regards Tinman