Several websites claim to enable you to learn Yoruba. Since Yoruba is a tonal language, it is particularly hard to learn without hearing it, so the best sites include audio clips of each phrase. You can learn the basic structure of the language, the alphabet, some pronunciation, and a lot of vocabulary for free online. The University of Georgia's African Studies Institute has published a series of lessons online (http://www.africa.uga.edu/Yoruba/index.html). Other sites, such as Roots and Rooted, include detailed information about the Yoruba alphabet and more resources for language study, including online dictionaries.
Online resources can help you start learning Yoruba. However, you can only achieve true fluency by conversing, preferably with native speakers. Once you master some of the basics of Yoruba, probably the best option (besides taking a class or traveling to Nigeria) is to make some international friends. Social networking sites are a great online tool. If you have a Facebook account (http://www.facebook.com) , you can search for new friends in Nigeria. Likewise, on Second Life (http://www.secondlife.com), you can virtually "go" to Nigeria and talk to native Yoruba speakers.