You get background information from reference sources, like encyclopedias. They are ideal for learning more about a topic or getting a good overview--you use them just like you use Wikipedia. Because they summarize information you generally can't use them in a research paper, but always check with your professor to be sure.
Many of the entries in these background sources are written by experts, such as philosophy professors, and they can be quite lengthy. Remember to check for recommended / related sources at the end of the entries, as these will often be prominent articles or books that will cover the topic in more detail.
These encyclopedias cover philosophy broadly and have long entries. They are the best place to start learning more about philosophical topics.