Many sources influence prejudice and discrimination but the very beginning of this development starts in a child's home environment. In some cases, the child will accept and continue the beliefs of his/her family. In some cases, the child will reject those beliefs.
Outside of the home, people are influenced by the neighborhood, the schools they attend, jobs, friends, extended family members etc.
Personal experiences also influence prejudice and discrimination. If a person witnesses something troubling, or just has a negative experience with a certain group they have preconceived notions about, then these experiences will just further reinforce their beliefs and prejudices.
Furthermore, the media (radio, print, movies, commercials, news shows, tv...) is extremely powerful. I think the media impacts us all more than what we may realize. The portrayal of certain groups has always historically been biased and known to reinforce social stereotypes and prejudices.