Hi Baniel! taralej here, I've been playing DKs since day 1 of WotLK's release, so I'll try to help you as much as I can in a noob-friendly way.
First of all, I'll explain the two resources a DK has, then I'll explain some commonalities.
DKs have runes and runic power. There are 3 rune types, with 2 runes from each type, with the whole set very often dubbed as BBFFUU. There is also a fourth type of runes called Death runes, which can act as any other rune and are generated mostly through talent. Each time you use a rune, you generate 10 runic power (RP), and each time you use a multi-rune ability, you generate 15 RP. Talents and glyphs can increase the amount of RP generated.
A lot of DK abilities are improved when you have applied diseases on your target, generally through (F) Icy Touch and (U) Plague Strike. This means that you will start almost all fights with these 2 abilities, no matter what spec you are. After that it depends on your talents.
Blood uses Heart Strike as a B ability and as the spec's main source of damage. Heart Strike is far superior to Blood Strike and completely replaces it. Death Strike is used as an FU ability for the healing sustain and to convert the runes into Death runes through
Death Rune Mastery in order to use more Heart Strikes. The spec's RP ability is Death Coil, but it is used as a last priority, after everything else has been used.
Frost uses Blood Strike as a B ability and converts the B runes into Death runes through
Blood of the North. The spec uses Obliterate, always combined with
Annihilation, as its FU ability and main source of damage. The spec's RP ability is Frost Strike, which makes up for a significant amount of its damage. Howling Blast is used on
Rime procs, when it doesn't cost any runes, or when there are more than 1 targets.
Unholy uses Blood Strike as a B ability and converts the B runes into Death runes through
Reaping. Blood Strike also triggers
Desolation. The spec uses Scourge Strike as its FU ability, which does a mix of physical and shadow damage. The spec's RP ability is Death Coil, which is a lot stronger than it is for Blood. Unholy can also raise a ghoul as a permanent pet, contrary to Blood or Frost which can only raise one for 30 seconds. Unholy's pet ghoul is also a lot stronger than the usual ghoul and it has multiple abilities, among which a stun (
Gnaw). It can also call a gargoyle, which will deal significant shadow damage, scaling from your attack power.
The best spec for leveling is by far Blood. It can kill without stopping, it can solo elites, it can kill 5-10 mobs and end up on 100% HP, it can tank dungeons with very little healing needed from a healer. Death Strike being an integral part of Blood means you do not sacrifice damage in order to heal yourself, unlike Frost or Unholy which have to sacrifice Obliterate and Scourge Strike in order to use Death Strike, which is weaker for them but stronger for Blood. Generally, you will start a fight by using Icy Touch and Plague Strike, followed by two Heart Strikes and a Death Strike. At this point you will be out of runes, which means you can use Death Coil and then wait for your runes to come back for more Heart Strikes and Death Strikes. Remember to always keep the two diseases on your targets, refreshing them just before they fade - Heart Strike deals increased damage and Death Strike heals for 5% of your max health (increased by talents) for each of your diseases on the target. Later on you will get Rune Strike, which you will find to be a lot better for spending RP than Death Coil because you can queue it while using your other abilities. As for glyphs, I strongly recommend getting
Glyph of Death Strike - it will make your life a lot easier while leveling.
If you have any further questions or I didn't explain something clearly enough, feel free to ask or just PM me.