I want to make my SDL game fullscreen but whenever i do it the screen flickers. Does anyone know how to make it not flicker?

You mean it flickers once when you change resolution? Or it flickers continuously when it's in fullscreen mode?

You can't do much about the first one; some fade to and from black before and afterwards but it's still usually noticeable (especially on a CRT).

You probably need to implement some form of double buffering; http://libsdl.org seems to be offline or I'd link to the examples there, but the easiest way in which to achieve double buffering is to blit everything to a 'back' surface of the same dimensions/depth as your 'front'/screen surface, then blit that 'back' surface to the 'front'/screen surface.