My 99 F150 RWD has the P0171 code for its running lean any idea where to start

This is a lean condition code, I'd recommend starting with cleaning the MAF sensor. I'm sure you can find a fairly detailed procedure for that if you google for it. There are also other possible causes for that code, I'd say follow the steps in this article to diagnose and fix: https://www.aa1car.com/library/ford_lean_codes.htm