I'm trying to make the line move when variable "g" increases

function _init()
  g=1
end

function _update()
  if (btn(2)) g+=1
  if (btn(3)) g-=1
end

function _draw()
  cls()
  circ(64, 64, 20, 7)
  x = 64 + cos(g()) * 20
  y = 64 + sin(g()) * 20
  line(64, 64, x, y)
  print(g)
end

If anyone has any tips for me, or a good sin() cos() tutorial, it would be a great help.

You've got the right idea there's just a couple things going wrong here.

g is a variable not a function so in draw you should have:

x = 64 + cos(g) * 20
y = 64 + sin(g) * 20

The second thing is that angles in Pico8 are numbers between 0 and 1. So 0 is 0 degrees and 1 is 360 degrees. Since you're increasing/decreasing g by 1, what you're actually doing doing a full 360 rotation each time.

Change update to something like this:

function _update()
  if (btn(2)) g+=1/360
  if (btn(3)) g-=1/360
end

That'll give you a change of 1 degree each time g changes.

This site has tutorials about trigonometry for pico8: https://demoman.net/?a=trig-for-games