I've been playing defense for about 25 years and I have coached a lot over the years also. All of the advice so far is good and true but defense is a philosophy that you need to understand so that you can play it well. You need to play defense on the offense. I don't mean be an offensive defenseman, what I mean is that you need to run the show, make the guy go where you want him too without him even knowing it. You don't react to them because then you're chasing, you make a move that makes them do what you want. Use the boards to your advantage, don't square up shoulder for shoulder with the guy, line up your outside shoulder with his inside one and give him the outside, he'll take it. Then crossover and turn and run him into the boards. A hip check is a good thing then but I definately can't explain how to do that on the computer.
As far as the open ice hitting, it's all about anticipating where the guy is going. My first suggestion would be to find a place to play where you can play forward. If you play it and understand how to play it you'll know what they want to do and where they're supposed to be. Then when you play D you'll know where people will be as the play develops. Find a spot you think the guy will be and be there when he gets there. Make sure your going forward, not backing up and definately not standing still. Most important thing in the open ice.......COMMITMENT!! Decide and then go, don't hesitate or you're done. You'll miss some in the beginning, but eventually it will just be a feeling and you'll just react. Good Luck!!