Opengl is a great starting place ... its been around for a long time and has tons of tools and libraries on top of the core. It isn't likely to go away anytime soon. It works on all the major PC platforms. The only downside that I know of to GL is it does not seem to perform quite as well as native windows (directx family), but it does well enough that many professional games have used it. You can't tell the difference unless really, really pushing the hardware.