Put SDL_GetTicks() (or something that changes with time) as the argument to cos() and sin(). Make it the same frequency and phase if you want a continuous looping motion. Also, multiply it by something big, so you can see it on the screen: cos() will be -1 to 1 pixels, 200*cos() will be -200 to 200 pixels.

A little bit of trigonometry is required here so I’d like to complement JonnyD’s answer. The basic formula for a circular motion is:

x = A cos(ωt)
y = A sin(ωt)

with ω being the angular velocity, i.e. 2π multiplied by the frequency (in Hertz or turns per second) and A the amplitude, e.g. in pixels. ω is a value that increases indefinitely so you need to keep a variable that increases with time. Both sine and cosine vary between -1 and 1 so you must multiply the result with a value big enough so that you can see the result on a display.