If you use your favourite search engine and search for how to play online holdem
you will find lots of introductory articles.
Start at the top hit and read a little of it. If any of it starts to make sense continue with that one.
If it doesn't make sense, or when you have finished the previous one, move on to the next one.
When you feel that you have a grasp of at least some of it register with a Poker Site
and play a few games of their 'Play Money' or find one of their freerolls
, if they have any.
BTW: I think that this should have been in the 'Learning Poker' section. So if you have any more questions after doing the above, I suggest that you ask in there.