Depends on the agency one has been recruited and also on the aspiring individual as one may be keen to work as a uniformed police office while another may have the inclinations to work in specialized forces like Us Drug Enforcement Agency thus getting into different training centers and modules.
Though there is no specific degree offered to aspiring students to get them into policing agencies but high school diplomas or degrees in a range of subjects from political science to criminology to chemistry may help to be preferred, though it solely depends on the recruiting agency.
http://www.schoolanduniversity.com/study-programs/criminal-justice/law-enforcement-police
http://www.trafficgeyser.net/lead/police