How to create a save function in love2d?

Sure. You can use various libraries. My current recommendation is Ser Binser (Ser is deprecated). This process is called "table serialization". Then you can do something like this to effectively create a "save".

local ser = require 'Path.to.ser'
local save 

function love.load()
    if love.filesystem.exists( 'Save.lua' ) then
        save = love.filesystem.load( 'Save.lua' )
    else
        save = {} -- Put settings in here.
    end
end
-- etc. etc.
function love.quit()
   love.filesystem.write( 'Save.lua', save )
end