The new death knight emerges
At level 58, assuming you're specced blood, you will have at least a few decent tools in your belt to start in as a tank. You'll have Death Grip, Death Strike, Heart Strike, and Pestilence by default, all good ways to keep, spread, or grab aggro or defensive power. If you spec for blood,
you'll be able to grab cornerstone talents such as Improved Blood Presence, Rune Tap, and Vampiric Blood.
That said, you're still missing a few cornerstones that make death knight tanks able to truly bring the pain. Death and Decay doesn't come until 60, and Dark Command won't come until level 65. Icebound Fortitude won't come along until level 62, while Rune of Swordshattering doesn't show up until level 63 (You can settle for Rune of Spellshattering in the meantime). Rune Strike, a massive threat gainer, won't show up until level 67. Even worse, Blood Shield, which helps a death knight absorb a lot of damage and is essentially our roundabout version of the blocking power that paladins and warriors get, won't be coming along until level 80.
The great tank gear drought of the 60s
The other problem is gear. Upon exiting the starting zone, you are thrust out into the world with a minimum of tank gear. Both your trinkets do have tank stats or procs, as does your chest, but it's still a mishmash, and you may find yourself wanting solid tank gear. If you're an alt, as you probably are, you may be lucky enough to have a main that can throw you a few of the new tanking heirlooms, including the cape, shoulders, helm, and chest. That said, the level 58 to 70 field is generally a wasteland of decent tanking upgrades.
Whether from smithing or quest gear, Outland really isn't that big on gear specifically for tanks, especially for questing. Your best bet is to spend a little time outside of dungeons questing in order to grab what high-stamina quest rewards you can, such as the Underworld Helm. You can also grab the Fearless Girdle and the Circle's Stalwart Helmet in Zangarmarsh, which you can gem with Wrath-era gems to give yourself a nice head start on your HP pool or tank stats.
Questions and queuing up
Ok, that's the bad news. The real question though, is simple: Can you still tank? The answer, essentially, is yes. It isn't always pretty, but low level dungeons are a lot simpler than high level heroics. Your gear doesn't have to be so severely tanked focused, either. That said, there are at least a few things you should do to make sure you're the best tank you can be.
Start with a tanking spec that focuses on the survival tools you need. I would recommend something similar to this blood talent spread. Since you're building a good tanking toolkit and you can't quite rely on gear yet, focus on the abilities that will make you as non-squishy as possible. You can start building on utility talents once you're in your late 60s or so. From here, I'd start getting Improved Death Strike as soon as possible, since the extra healing will be invaluable.
Queuing up as a level 58 tank, you may get an old world dungeon to start -- on my new level 58 death knight, I got Upper Blackrock Spire. That can be a small blessing, as old world dungeons tend to be slightly more forgiving as far as the HP and damage potential of mobs, and you have a decent chance of getting through everything with a minimum of trouble.
As you run your first few dungeons, your biggest banes are going to come in the lack of Dark Command for a quick taunt and the lack of Death and Decay for a solid, sustained aggro grab. Luckily, in both these cases, you can generally make do with Death Grip for taunting, and Blood Boil or Heart Strike for keeping aggro on multiple mobs. Just be aware that Death Grip does have a daunting cooldown, and once it's done, all you can do is attack the mob and hope it aggros back on to you before it kills whoever it fixated on.
To keep aggro as best you can, you may need to open with a Blood Boil (to be replaced by Death and Decay once you get level 60) once all mobs are in range. This will allow you to wrangle in multiple enemies before AOE happy DPSers get their hands on them. That said, It's generally a good idea to at least try to get them to follow a basic kill order. Tell them which mob you want them to kill first. If they can do that, it gives you the time to pull everything in and get diseases up and spread via Pestilence using that one mob. That not only gives you some nice ticking threat on every mob, but Frost Fever and Blood Plague offer debuffs that will help you live longer. Also, be sure to always stay in Blood Presence. The extra threat, survivability, and rune regeneration it gives is indispensable.
Once diseases are up, you'll primarily use Death Strikes and Death Coils (replace Death Coil with Rune Strike as soon as you get it), with Heart Strike used on 3 or fewer mobs when you have blood runes, and Blood Boil used on 4 or more mobs. To get a better idea of how to build a good blood tank rotation, check out our blood tanking 101 guide.
Communication with your group members is also important. It can help to make sure they know that you're missing a couple little tools, and that if they follow you closely and use a kill order, it can avoid some mishaps. Mind you, not all DPS will like doing that. If you're queuing for PuGs, you may even get some people dropping group or trying to kick you. Hopefully, if you're leveling with buddies, that won't be a problem as much, but it's worth it to be prepared for negative Nancies if you're taking on any random people.
The good news is that experience comes quickly in dungeons. If you're fully rested or if you have heirlooms, it should only take you 2-3 dungeons to hit level 60. Once you're there, you'll have Death and Decay and keeping aggro will become a whole lot simpler, especially if you can glyph for it. From there, just keep at it. Be sure to train whenever you get a new tanking tool, such as Rune Strike or Dark Command, and start working it in where needed. When you get Rune of Swordshattering, go apply it.
Looking to the future
Once you hit 70, you should be in excellent shape. Not only will you have almost all of your major threat and defensive tools other than Blood Shield, you'll be high enough level to equip Cobalt armor. This "set" isn't a true set, but it's something blacksmiths can make that provides a good amount of dodge rating and stamina for every plate armor set. Your healers will thank you for obtaining a set for sure. You can also upgrade to Tempered Saronite armor in your mid-70s, putting you in a pretty good place when it's time to start running those Cataclysm dungeons.
Learn the ropes of endgame play with WoW Insider's DK 101 guide. Make yourself invaluable to your raid group with Mind Freeze and other interrupts, gear up with pre-heroic DPS gear or pre-heroic tank gear, and plot your path to tier 11/valor point DPS gear.