It seems like everyone is saying 'finna' these days, but the expression actually has its roots in African American Vernacular English (AAVE).
For example, if you tell someone that you're 'finna take a walk around the block,' what you mean is 'I'm going to take a walk around the block.' 'Finna' is an abbreviation for the phrase 'fixing to.' It is essentially used in place of the more common (and the more easily understandable) 'gonna,' which is short for 'going to.' If you're finna do something, you're going to or planning on doing it.
And it actually has nothing to do with fish. Sometimes, though, I realize I've still got it - like last night, when my father asked me, ' What does 'finna' mean?' This one's actually been around long enough that it was cool when I was in high school, so I could handle it.
It's almost not worth keeping up with what the cool kids are saying these days, because as soon as you learn it, they'll have moved on to shorter, more complex abbreviations. Slang changes as quickly as fashion does (or as quickly as Heidi Klum claims fashion does on Project Runway): One day you're in, and the next day you're out.
