Here is how (I think, if I remember well) it works:
int SDL_BlitSurface(SDL_Surface *src, SDL_Rect *srcrect, SDL_Surface *dst, SDL_Rect *dstrect);
You have to create a SDL_Rect and specify the x and y position and the width and height of the sprite on the sheet.
SDL_BlitSurface(thespritesheet, &theRect, thescreen, NULL);
(NULL is for another SDL_Rect, which indicates the position of the sprite on the screen)
I don't remember if it works like that but I think so, hope it helps you...