The Big "O"

So I have an issue I can't get what everyone talks about. Orgasm. That's all I want. I've watched porn to see what it's all about and I just dont know how to get it. I clitorical stimulate while masterbation and I start twitching I guess you would call it and I can't handle any more so I stop and my legs shake for awhile after. But nothing happens like what it should. 
Someone please help.