Have a look in the Guardian/Defender sub-forum under Self Help.
There are 3 stickied guides there that should answer most of your questions. Just keep in mind that Avyn's guide was written when the maximum level was still 60. All the general principles still apply, but some of the absolute numbers are a little low.
If you still have questions after that, post in that sub forum.
Playing a defender isn't for everyone, but it can be a lot of fun