if you want to start easily use OpenGL. It's really easy, but you don't have all the possibilities you can do using Direct3D IM.
Some coders prefer OpenGL, some D3D. Both are really good API's. I learned D3D because I wanted to use DirectSound and DirectDraw in my actual project, too. And that's an advantage, the architecture of all Direct-API's nearly the same, so stepping from one to another is not that hard.
You can get good results with both API's, have a look at Half-Life or Quake 3, both using OpenGL, HL looks even better in OGL in my opinion.
CU
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Skullpture Entertainment
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