As Friz mentioned in his lovely guide on shield tanking, there are three ways of defending your ship from damage using a tank. These are the items that will help regarding armor tanking:
Active Tank: Uses hardeners and armor repair systems to keep the tank alive. Can use lots of cap if skills aren't good enough.
Hitpoint Tank: Uses armor plates to add lots of HP to your armor. I wouldn't recommend you use this on it's own, only in conjunction with either an active or passive tank.
Passive Tank: Damage controls, energised plating, and resistance plating all contribute to this tank.
Just a note, I've only listed armor repairers in the active section, but they should be used for any tank. You will die very quickly without one and find yourself in a pod very quickly.
Now to list recommended items, with different 'variants' in italics underneath (best to worst), just in case you pick one up and don't know if it's valuable or not.
Armor Hardeners: Raise resistance to one damage type by 50% (55% for T2). These need cap to run.
T2, N-type, Radioisotope, Microcell Nanite, Voltaic Nanite, T1
Armor Plates: Add HP to the armor. No cap needed. Slows ship down by 10m/s for each one fitted. Come in 50mm, 100mm and 200mm (for frigates), 400mm and 800mm (cruisers) and 1600mm (battleship) flavours.
Rolled Tungsten, T2 (due to fitting reqs), Crystalline Carbonide, Titanium, Nanofiber (only slows ship by 5m/s), T1
Armor Repair Systems: Repair a set amount of armor per cycle. Come in Large, Medium and Small. Note: The 'Reprieve' T2 repairers are listed, but are not yet in game. They have the same skill requirements as normal T2 repairers.
T2, 'Accomodation', Automated Carapace, I-A Polarised, T1
Damage Controls: I'll be honest, I've never used these, so I can't really say which is best. They give bonuses to both Armor and Shield resists, so could be quite useful.
Energised Plating: There are 2 different sorts here, passive hardeners that resist one damage type (32.5% for named/T1, 37.5% for T2) and use no cap, and regenerative membranes, which increase the armor hp by a set percentage (12.5% for T1 and named, 15% for T2).
(Same variants as hardeners)
As an added note, also in this group is the almighty Energized Adaptive Nano Membrane II, which gives 20% to all resists, more with compensation skills.
Resistance Plating: Similar to Energised plating, but gives less of a resist and only uses 1 grid. I've never used these, I'd go for Energised plating any day, you should have more than enough grid to fit them.
As for skills, try to train the following to get an effective tank.
Hull Upgrades: Essential to be able to use pretty much any of the above. Grants an extra 5% to armor HP per level. Train to level 5 to be able to use T2 hardeners.
Repair Systems: Lowers the cycle time on armor repairers. Each level lowers cycle time by 5%. Train to at least level 4.
Mechanic: Anyone who flies an AF will have this at level 5 anyway. Gives 5% more hull HP per level. Train to level 4 to use medium armor repair T2, level 5 for large.
Armor Compensation: There are four of these skills, one for each damage type. They work to boost the resists of armor hardeners and plating, 3% per level for active hardeners, 5% per level for passive.
Also try and train up some cap skills if you haven't already. And use Nos if you're still struggling. Running a tank and guns at the same time can be very taxing on the old capacitor. I'm now up to the point where I can just about use tank, guns and AB with a Nos active.
If anyone is still struggling, and you want me (or anyone else) to try and work out an effective setup for you, just post below and I'll have a go.
Done! (Happy now, Lee?)
beaten me to the finish line. I'll finish mine soon, then we can combine them together and make it a nice neat little package. Anyone wanna do structure tanking? 
Better to fit some reinforced bulkheads instead.