There are a few websites that will get you started on the basics of the Cantonese language. Learn Cantonese (cantonese.ca) has a number of vocabulary lists and guides to simple grammatical structures. Vocabulary lists range from animals and bedroom items to movies, famous people and slang.
Another site, Cantonese Help Sheets (cantonese.sheik.co.uk) has an extensive archive of free flashcards, tests, learning aids and a Cantonese dictionary program called CantoDict, with native speakers pronouncing Cantonese words. Chinese Lessons (chinese-lessons.com/cantonese) provides free lessons, including comprehensive lessons on pronunciation, grammar and basic communication, such as telling the time and common greetings.
Cantonesepod.com offers free podcasts that teach you Cantonese terms and phrases. If you are a more advanced learner, you might consider using the BBC's traditional Chinese website (see Resources), which allows you to read BBC articles translated into traditional Chinese characters and listen to BBC radio broadcast in Cantonese. Station RTHK (see Resources) also provides radio programs from Hong Kong, so you can listen to a variety of Cantonese-language programs, many of them oriented to teens and kids.
Successfully communicating with a fluent speaker of Cantonese is an important step in learning process. Go to Italki.com and create a free account to find a language tutor (who will probably charge you a nominal fee for classes) or a language exchange partner (who will charge you nothing). Livemocha.com also has a supportive, enthusiastic community of users, who want to connect to language partners. If you want to find a group of Cantonese speakers and learners to speak to in-person, find a language exchange group through Meetup.com.