During the First World War, close to 6 million men served in the British Army. Men from all of the British Isles and her Empire answered the mother country's call for men. Roughly half of those were volunteers, the remainder were conscripted. Below are a few links to introduce the key themes involved in the recruitment of Britain's army in WW1.
A short movie from the BBC - http://www.bbc.co.uk/history/interactive/animations/wwone_movies/index_embed.shtml
An excellent overview from 1914-1918.net - http://www.1914-1918.net/recruitment.htm
Pals Battalions - http://www.bbc.co.uk/history/british/britain_wwone/pals_01.shtml
The Derby Scheme - http://www.firstworldwar.com/atoz/derbyscheme.htm
The Military Service Acts - http://www.1914-1918.net/msa1916.html
WW1 Recruitment posters here