There are many sources to find the answer to Plato's philosophies, such as the Stanford Encyclopedia of Philosophy's webpage on Plato or the Encyclopedia Britannica's entry on Plato. Answers can also be found in university-level philosophy textbooks, or within Plato's original work.
Encyclopedias are a great resource to find a summary of the life and viewpoints of Plato. Encyclopedia Britannica is a general encyclopedia that has both this information both in print and online. The Stanford Encyclopedia of Philosophy is a resource from Stanford University that focuses only on philosophy. Both of these resources provide the best summary of information about Plato. They are more credible than open encyclopedias such as Wikipedia because the authors of the contents are experts.
However, for those looking for some more in-depth research, a better option is to consult a philosophy textbook, or read the actual works of Plato. Many university-level textbooks cover Plato and may provide a better explanation of his philosophies. Additionally, his many classic works are available both in physical book copies available in bookstores, or for free online from sources such as Project Gutenberg. By reading his dialogues, a person can have the most in-depth and complete understanding of his philosophies and how he arrived at his conclusions.