I would like to know what software I should use for making the game. I am planning to make a very basic 2D fighting game but which uses input given by a load cell.
please reasearch your question before asking on stack exchange. your question could easily be solved with some google searches.
On to your question:
python is a programming language that is the main, most popular language on raspberry pi. pygame is a library, or module, for python that allows the creation of simple 2d games. GPIO (General Purpose Inputs/Outputs) are the pins built into the raspberry for interfacing with sensors, which you can control with a simple python module.
I would recommend first trying out some basic python if you don't know it already. just do a google search for 'basic python tutorial', or something like that. then learn how to use pygame and make your game. finally, learn how to use the RPI.GPIO module, so you can incorporate sensor input into your game.
hope this helps!! feel free to ask more questions if you need help.