One word at a time, ten thousand times.
I took course class
I learned Spanish in school. You can buy "Rosetta Stone" if you want to learn on your own, or take classes at a local community college or whatever.
I think I'd try immersion. I'd like to spend a year in Mexico learning Spanish, but due to budgetary restraints...
Either take classes both in and out if school or go to the country and plunge right into the deep end, you'll end up having to learn the language if you can't survive without doing so.
Join a class
live in the country