WARNING! WALL OF TEXT INCOMING!

General advice:
Buy a palette. Start mixing colors now. Don't look for recipes, just mix mix colors and see what happens. This will help you learn how to get exactly the color you want, as opposed to looking at a color you like and going, "Which GW color is that?"

Paint diluted. Use water to thin your paints right off the bat. Thick paint obscures detail. Thin paint can be layered.

Don't follow a certain painter or blogger. Get advice from as many different sources as you can. Personally, I don't find youtube very helpful, I expect you won't either after you get ahold of the basics of how brush touched figure. Dakkadakka.com and coolminiornot often have excellent painting articles, tutorials, and advice.

Don't waste your money on GW tools. Brushes, paint, glue, modelling supplies, green stuff, all of this can be found cheaper or better quality (usually both) elsewhere. I'm a big fan of the Vallejo paints, but they've recently had a small price hike. They're still better quality than GW paints though. And they come in droppers. Droppers make it easier for you to mix paints. I'll put it like this. I have a whole drawer of Vallejo paints that I haven't used in a year. All I had to do was shake them up and they were fine to use. If these were GW paints, several of the pots would have lids stuck on, several would have semi-dried out to the point that I would have to add water and carefully re-mix them before I could use them. Several more would have dried into a solid, unsaveable mess. It's not fun to have your painting project interrupted because you have to go buy new paints because GW can't be bothered to sell a quality container.

As soon as you have the money, get yourself a Holbein Solid Brush Cleaner. It looks like a slab of soap in a hockey-puck shaped container. Then get yourself a good quality natural bristle brush, like a Winsor & Newton Series 7 sable. Paint with the tip, don't dunk it into the paint deep enough for the paint to touch the ferrule, and meticulously clean your brush every time you use it. It will last you far longer than any synthetic will, and it's impossible to do good detail work when you've got a plastic brush where three bristles are pointing off at 60 degree angles from the core.

Airbrushes are finicky creatures. Be very careful about getting them. I ruined my first airbrush (cheapo Testor air-in-a-can model) on my first try. I ruined my second airbrush (more expensive Mr. Airbrush with tiny (and silent) compressor) after getting about a week's worth of use out of it. Airbrushes have to be cleaned meticulously, and have many small moving parts that if broken ruin the device. Because of all the setup and cleanup involved, I would not say that airbrushes really save time unless you use them as a very expensive base-coating tool. I personally don't think airbrushes are that good for saving time, what they're good for is tricks that traditional brushes can't do. By all means, if you have the money experiment. But I think if you're saving time, you're either painting a massive army of space marines all one color or you're not cleaning your airbrush properly and risking damage to it. Like I did.

And finally, consider moving away from GW. If you aren't into playing the game, then there's no reason to stick with them. By all means, buy GW miniatures if you like their aesthetic, but I tend to find GW minis are clumsy, poorly designed, and nowadays with finecast poorly manufactured. They call themselves the Porches of miniatures (and charge accordingly) and then try to pawn off on their customers inferior models. Their plastics are still good (a lot of the time anyway) but they are not the only game in town, and if you hang out in GW stores they will try to hide that fact from you. Depending on what sort of miniatures you enjoy painting you can find just about anything on the web, but two of my favorite companies right now are:

Red Box Games and
Infinity

They do much more reasonably-proportioned miniatures than GW, often at better prices. Red Box Games is particularly excellent quality-wise. Sculptor Tre puts an astounding amount of thought and care into each and every sculpt. Compare a GW and a RGB mini sometime and you will see the difference. Every miniature from RGB looks like a figure with weight, and when they're in a battle pose they look reasonably like a body in motion. GW loves to cover minis with rediculous trinkets and details, like glorious soldiers are carrying an entire knick-knack shelf on their back on the way to battle. Tre from RGB covers his miniatures in loving detail, but it is always exactly what is appropriate for the model.

Other companies that I think do spectacular work (but whose minis I've never handled personally) are:
http://studiomcvey.blogspot.com/
http://www.maelstromgames.co.uk/index.php?act=cat&cre=min-blg (Great source for monsters from their Banebeasts line. Be warned, they aren't afraid of sculpting giant monsters that wouldn't care to clothe themselves to be anatomically correct. Be prepared for minotaur bollocks.
http://www.avatars-of-war.com/eng/web/index.php?option=com_content&view=article&id=59&Itemid=54 (Avatars of War stick with a more GW-esque aesthetic but manage to usually do it better than GW. Good source for fantasy heroes.
http://www.spartangames.co.uk/games/firestorm-armada (Spartan Games doesn't always impress me. I strongly disagree with them showing CGI sculpts as opposed to actual models in some of their advertising. Their Uncharted Seas line are often underwhelming and I have no idea what their rules are like, but their Firestorm Armada has some of the best hard sci-fi ships available. And some of their new designs look very promising- if only I could see them in the flesh as it were.)

But the most important tip: Just do it. Your first few miniatures will likely be underwhelming. Everyone goes through that phase. The only way you get out of it is to keep at it. One advantage to starting out with metal models as opposed to plastics is it's super-easy 5 years down the line if you're embarassed by your old metal minis to strip the paint off of them and do them over.

And with that in mind, if you're looking for cheap minis, you might try ebay. Just make sure you buy metal rather than plastic. Plastic can be stripped, but it's harder, and primer often "stains" the plastic.

SckizoBoy:

Zerstiren:
There are pictures of a "blood-letter" getting a glaze. It describes a major difference . . . when I didn't see anything. So no glaze on my marines. Good notes!

I personally dislike glazing, because the effect becomes too shiny and fake/plastic.

However, prodigious use on Bloodletters would work, bringing out an otherworldly sickliness to them that sort of suits them... just don't do it to the face! As for Marines, though, glazing (IMO) never suits them, because they're supposed to have either the down and dirty battered look (hence the metal highlights to show chipped paintwork and battle damage) or the practical disciplined look of clean burnished armour. If it's shiny, it doesn't come off that well on miniatures, so you can only get that effect by layered and contrasted washes (AFAIK the model for the Dark Angels Terminator captain in the codex has about thirty-odd layers of paint to achieve that sort of shimmering faint green/bleached bone sheen to it).

Very much agreed.

And this is partly why I've moved away from GW paints. They seem to have a slightly glossier finish than most other companies' miniature paints. And any more the only things I want to look glossy on my minis are things that are supposed to be glossy.

GW's washes however remain the best I've ever used. Especially Devlan Mud, or as it is known on dakkadakka, Devlan Miracle. For a new painter, a quick wash of Devland Mud is an almost can't-fail way of bringing out detail and muting colors .