Then use JEDI-SDL. Here is wiki on the subject:

http://www.freepascal.org/wiki/index.php/FPC_and_SDL

Here is link to latest JEDI-SDL:

http://sourceforge.net/projects/jedi-sdl

Here is SDL docs:

http://www.libsdl.org/cgi/docwiki.cgi/SDL_20API

You will want to use the SDL_SetVideoMode function with the SDL_OPENGL flag, with the appropriate width and height parameters, and if you want full screen you will pass the SDL_FULLSCREEN flag.

GLUT is simpler to use, but does not have as many features.