Well now you have hit 80, if your not a fan of leveling then I'd say this is where the game will really start for you and you can have a lot of fun!
It depends what you want to do really. You can be a raider, group with people and go and conquer raid dungeons, or you can be more of a PvP'er. I guess you can even do both if you want. The best advice I can give is the same as Theroin, just try everything and soon enough you'll learn what you enjoy and what you dont enjoy.
If you decide you want to raid then 1st of all you need to decide what role you want to play in the group and with a DK you get options:
1. Tank - The guy who basically protects the group, takes the beats, makes sure he has aggro on mobs, allowing DPS and healers to do their job safely. If this is what you want then from the sounds of things your gonna need to upgrade your gear. I'd say start by scouring the AH for Tempered Saronite gear. As said above this is a decent bit of gear at a good price to start you off in the right direction.
Run normal instances 1st to try and get upgrades on your gear and also learn how to tank properly, learn what your moves do, learn to keep aggro on your targets. What you want to aim for 1st is getting your deffence up to 535 for Heroics (or 540 for raids) this will make you immune to being crit, a real must have for tanks. When your deffence is that point, as a DK you really want to work on +hit, prefferably without gimping your stamina too much though. Then get yourself into heroics to get better gear and progress to raiding standards. If it doesnt go well to start keep at it, tanking isn't easy to start so just keep your chin up and always come back here to seek advice.
2. DPS - One of the guys who deals out the damage in the groups, takes mobs down. Again if this is the way you want to go there is a cheap blue set you can buy from the Ah again (made by blacksmiths) and you want to focus mostly on +hit, str and attack power. Research the different specs and builds, try them all out if you want to see which one you find more fun or which provides you with the best results.
Obviously that then leaves you with PvP in the forms of Battle grounds or arena. This is something I'M not going to comment on as I've never been a fan, so I know nothing about it. Not enough to advise anyway, especially from a DK point of view.
Always feel free to ask questions, there are some really helpful people here who know what they are talking about. Theroin is one and another is a guy called Dilbert who helped me a lot. Good luck anyway, hope you enjoy your new DK, come back and tell us what you chose and how it goes :)