what's a dew layers? o.O;

Few layers. Depending on the darkness of the color and thickness of the paint, 2-5 layers should cover the black.

oh ok... -is starting to reconsider painting her space marines black-

She's spot-on here, it's a pain in the ass. For example, my army's primary color is white. They've got white power armor with black shoulders, gold trim and insignia, and red cloth (as well as red helmets for sergeants). Blue eyes.

I do the following.

I assemble the entire model except for the gun and backpack, I leave those off. The exception is conversions and particularly tricky models - for example, I'm in the process of turning a Dark Angels veteran squad into a Storm Crusaders (my own chapter) Sternguard Veteran squad. They're wearing robes, and they're in more imaginative poses (and have a few extra accessories) than the rest of my Marines, so I went ahead and assembled them to make sure they'd fit together properly.

Once I've assembled my models, I start with a base coat of white (Skull White). A lot of the gray shows through, but this coat is mainly to make the rest of the coats stick. I then apply a second coat of white to everything that won't be another color... in other words, to all the white armor, but not the shoulders or trim. Then I paint everything that's going to be another color black (Chaos Black), with the exception of the rebreather and the eyes, which remain white. So the shoulders, weapon, backpack vents, trim, armor separations, cloth, and electronics all get a coat of black. Then I apply the red (Red Gore). If there's any cloth, it gets two coats of red, as do the weapon trims. If it's a sergeant, the helmet gets two coats of red.

Then I do another coat of black over the electronics on the helmets, because they're going to be painted gold (Shining Gold). Then I paint the trim, insignia, backpack vents, and electronics gold. Then the weapon frames and rebreather get a coat of silver (Boltgun Metal). The eyes are painted blue (Ultramarines Blue), and the purity seals are red (for the wax seal) and Bleached Bone (for the parchment). If there's any terrain on the base, it gets painted Desert Yellow.

I then go over it and any color that's found its way onto the white armor and touch up with more white. Then I gently scrape off any paint at the contact points on the weapon and backpack, and glue them onto the mini. After a final round of touching up, the base gets a coat of Chaos Black, and it's finished.

I'll take some pictures of my army so far so you can see.

blarg DX $200 just to start and that's not including paint (though it is including paint brushes and a "how to paint" guide book >_>

is it possible to use different units in 1 army? i.e. if i wanted to use a space marine army, but use a chaos space marine unit in that army >_>.
or a necrons unit

you should paint them pink....

Not as far as I'm aware. The only way to do that (that I can think of) would be to create two separate armies and field them both at the same time, e.g. a Space Marine army and an Imperial Guard army, but your opponent would have to be allowed to either do the same or double the size of his army to compensate. I have seen cases where two players team up in the same game against two other players.

Hell no.

but your dp looks so pretty

First of all, I apologize for the quality of the photos. I am not a photographer. Quite the opposite, I suck at photography. But here's my army.

HQ

Knight-Captain Torvus Videar (Captain). Terminator Armor. 140 points. Angle 2.

Saint Claudius (Chaplain). Jump Pack. 115 points.

Prophet Chronos (Librarian). Epistolary. 150 points.

TROOPS

Knights of the Chapter (Tactical) Squad Castor. Meltagun, Heavy Bolter, Power Weapon. DT Rhino. 225 points.

Knights of the Chapter (Tactical) Squad Pollux. Meltagun, Missile Launcher. DT Rhino (not shown, don't have it yet). 210 points.

Angels of Retribution (Scout Sniper) Squad Redrick. Camo Cloaks. Sergeant Telion. Sniper Rifles. 137 points.

Squire (Scout) Squad Lobo. Heavy Bolter. 85 points.

ELITES

Holy Paladin (Terminator) Squad Mordecai. Assault Cannon. 230 points. Fielded (as shown) with my Captain.

Honored Paladin (Dreadnought) Remus. Extra Armor. Assault Cannon. Missile Launcher. 140 points. Angle 2. Angle 3.

Steadfast Paladin (Sternguard Veteran) Squad Adamos. Plasma Gun. Meltagun. 140 points. Typically fielded (not shown) with my Librarian, but not always. If the Captain and Chaplain can be fielded, my Librarian stays behind (two HQ limit).

FAST ATTACK

Holy Martyr (Assault) Squad Talonus. +5 Marines (unpainted, so not shown). Power Fist. Combat Shield. 220 points. Fielded (as shown) with my Chaplain.

Tornado pattern Land Speeder squadron. +2 Speeders. Multi-Meltas. Tornado Pattern Assault Cannons. 300 points. My fast-moving tank-killers.

HEAVY SUPPORT

Predator Annihilator "Sudden Death". TL Lascannon. Sponson-Mounted Lascannons. Storm Bolter. 175 points. My mobile heavy-weapons platform.

All together, 2267 points.

I'm planning on getting a Predator Destructor and a Land Raider Crusader to flesh out the Heavy Support slots. I'm not sure yet what I'll do with my third Fast Attack slot.