Use DirectX or OpenGL?

[Gameman]

I am because I don't know what to use OpenGL or DirectX?
I heard some people saying I can use both.
How is this?
what is better for online games?
And when I choose I will need some tutorials too :).

[Gameman]

Thanks anyway I knew it I will use directX11 and learn this tutorials-------------------->http://www.rastertek.com/