Locking the framerate

Look into Fixed-time stepping

Don't know if this is the best way to do this but just put a framebreak in your gameloop like this:

unsigned int framestart;
#define WAITTIME 20

at the start of your loop:
framestart = SDL_GetTicks(); (or whatever timer you're using)

at the end of your loop:
while((SDL_GetTicks()-framestart) < WAITTIME);