Get involved with everything, especially any public events your school does.
Always be helpful and polite.
Get reasonable exam results.
Try and have an extra curricular area you are particularly good/involved in - drama, music, sport, public speaking etc.
My friend got voted as head boy. Well in our school, the candidates have to do a speech in the staff room and the teachers get the final vote.
I'd say you'll have to be very hard working, nice, friendly, confident, good at making speeches as you will be asked to do loads representing the school. Try joining clubs or societies to get involved with people in other classes/years . :)
I guess it'd look pretty good on your CV and teachers usually look for people with good grades as well as you will be a role model for younger students :) hope this helps
Get involved with the Student Council! If its a staff vote get well known among the staff and get noticed by the Head Teacher. If its the Students vote observe what issues they have in school and see if you solve them. Also tell them about the ideas you have that they will benifit from if you become Head Boy. Make sure you dont say - its for my CV - cause then they'll think tis for the wrong reasons... its important to want to do it for other reasons such as it would be rewarding :) This along with the other stuff already mentioned will give you a good chance :) Hope this helps...