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.

Invented in Borat's movie. It is said, when something great happens.

fixing to- verb meaning to start or begin

Generally used only in the state of Texas, 'fixing to' is the equivalent of 'about to' or 'getting ready to'. If used outside of Texas, likely the person using it is from Texas or has spent a great deal of time in Texas. Also, this person will likely be on the receiving end of, "You're... what?"

going to or about to. Used only in the deep south or by people from the deep south.

Also pronounced as "fittina".

when southerners are about to accomplish tasks. pronounced "fiddin ta"

a fuck ass way for people from the south to say about to