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.
I am fixing to take my shower. Anyone need in the bathroom before i do?
or
Dude, watch this, Rocky is fixing to kick Appollo Creed's ass.
or
Im fixing to go out , so don't wait up for me.