a lot from TV at first; i was early to learn to read (before school taught me), and i watched discovery and national geographic a lot (when the shows were still interresting). those were in english and subbed to my mother tongue. that is how i learned to understand (and, by extention of muscle memory, speak) english. becouse my mother tongue has simular script and vowals to english, i was able to read decently by reading the words out-loud, phonetically.
this resulted in the interesting situation that i spoke, read and even slightly wrote english by the time school started teaching me to read in my mothers tongue...
i honed my skills later on by gaming, both offline at first, later progressing into online and MMO (text-only first, later i started using voice chat).
this way i have grown to be fully bi-lingual, and i even think in english, just as my internal dialogue