It means you are about to do something, or more accurately you are doing something now that will lead to you doing the thing that you are fixin' to do. Fixing = preparation
Others here have the pronunciation wrong, when spoken it sounds
like one word: "fixina" which led to it's child word
finna
In the South we like to run our
words together, it's too hot to pronounce every letter.
Texan: "I'm fixing to go unload my pickup
cuz it's full of junk and I've gotta go get cokes for the
party."
Translation: I am in the process of preparing to go begin unloading the miscellaneous items from the back of my truck because I must go purchase beverages for the
party.