Many WW1 service records were destroyed by bombing in 1940. Those that survive are held principally by the National Archives in Kew. Those that have been digitised can be viewed online on a pay-per-view basis. The National Archives website has a detailed on-line guide to how to go about it.
The starting points for researching an individual are their name, specific service number, and the corps, regiment or unit to which they belonged. The service number may be found inscribed around the rim of WWI medals. The service number of soldiers and members of auxiliary corps can also be found in the Medal Index cards held by the National Archives. These can be searched on-line (series WO 372).
The Medal Index cards will allow you to find an individual’s corps, regiment and battalion or unit by copying the entry in the ‘Roll’ column and using that to search campaign medal rolls (series W0 329).
The principal record series for individual service records are:
Army, Royal Flying Corps. (WO 363, WO 364)
Navy (ADM 139, ADM 188)
Nurses (WO 399)
Women’s Auxiliary Army Corps, Queen Mary’s Army Auxiliary Corps (WO 398)
The day-to-day service operations of soldiers can be found in the official diaries kept by each battalion. Many have been now been digitised and copies can be downloaded (WO 95)
Good luck!