I'd say probably the same as learning any instrument: find a good teacher or place to take some lessons, especially if you don't have any previous experience with musical instruments. read more
If you want to play in a band mostly by ear, the regular layout of the chromatic button accordion is a boon. If you play from score sheets (like in an orchestra), the piano button keyboard matches the notation better. The mismatch of "controls" with notation makes tablature quite popular for amateur guitar playing. read more
Start learning songs. Once you have learned to play the left and right sides of the accordion together you are ready to begin building a song repertoire. The best way to learn is to have another accordion player show you some tunes. This is the traditional way that Cajun and Creole accordion players learn their songs. read more