Generally, Mandarin is spoken by northern chinese people. Cantonese is spoken in southern china such as Hong Kong and Guang Zhou and even in Vietnam.
Mandarin is more widely spoken in America, China, and just worldwide.
Mandarin and Cantonese can both be learned easily if you learn one or the other since they are somewhat like cousin languages.
Starting out though, Mandarin would be a better choice.
To learn Mandarin, this site can help with vocab and pronunciation:
http://pinyin.org
click on "dictionary" and there you can search for characters and pinyin. there, you can also hear the correct pronunciation