I very recently started the hobby and I have learnt a couple of valuable lessons in my short time painting and modelling. These are some of the things I wish I had known before I started. Since I am still new I will continue to add small things to this article.
1) Cutting Models from the Sprue.
If a model has a "thin/small" bit attached to the sprue and a "thick/big" bit attached to the sprue, cut the THIN bit first. When cutting one of the Right Arms of my Dire Avengers, I cut the BIG side first, the shock/pressure/vibration from the thick portion coming loose, snapped the thin part in half. So I was sitting with a Avenger Shuriken Catapault split in two! It should seem logical, but sometimes when just starting out, you are more excited about getting the models off the sprue than being careful doing so.
2) Paint THEN Glue
If it is logical for two pieces of a model to fit together and not leave any "Paint Required Here" bits in a difficult to reach place, then by all means go ahead and glue them. Example of this would be the front and back of the Dire Avenger breast-plate-armor-thingie. You won't be painting between the "back of the front, and front of the back armor". But once the armor is assembled, don't glue on the Head/Helmet. That makes it VERY hard to paint the top portion of the Armor, since there is now a Head where there should be some paint. It's not always easy to identify which parts are OK to glue before painting, but when you are glueing, just keep the painting in mind.
3) Be SURE of the color
Paint stripper burns. So don't get the color wrong. I ripped open my Fire Dragons, put them in the spray box and spray painted them. Waited for it to dry. Put them down and proceeded to fill them in a fun black and white color scheme. I finished painting the exarch and his 5 buddies. It looked amazing, but it FELT wrong. I looked at them wishing they really had some yellow, orange or red(being Fire Dragons and all). Since they were metal I decided to start over, nipped to the hardware store, bought a bottle of paint stripper, and dunked my little guys. After a couple of hours I set out with my old toothbrush and started scrubbing the black and white off. Every bit that splashed onto my arms or hands burnt like a...nvm. Believe me it hurts.
4) Use gloves
If you are using paint stripper, get gloves to work with. As I type this I can feel the sensation returning to my fingers.
5) Get proper help
My wife has been helping me clip my Eldar Grav tanks off the sprues. There are multiple options with a single grav tank box, so cutting every piece of the tank off the sprues and leaving them in a big heap doesn't really help, since now you have to dig more the get the pieces that are required to build the "version" of the tank you wish to have. I am not saying my wife isn't proper help, she just has no idea what the difference between a Falcon and a Wave Serpent is. If friends/family/spouses are intent on helping, try explaining what you want to do first. One of two things happens, first they get bored since it's no longer just "Cutting Stuff Up". Second, they start to understand what you are doing, this really helps. It's no longer just toy soldiers.
6) Have the basic tools
(thanks to all the forum posters that helped me compiile the list of things I required)
Have all the basic tools. It makes a big difference. Since I started, the only thing I wish I had is the hobby drill.
Side Cutters (GW works best, since they are a bit smaller than the standard one)
Files - Triangle/Flat/Round (I have used each of these since I started)
Tweezers (These really help to hold those small objects)
Citadel Foundation Paints
Thin, Thin, Thin paint brushes.
7) Have enough Paint
Last night I was painting and I really wished I had a couple of extra colors. I know when starting you prefer having more models rather than more paint. But it really sucks if you don't have the paint.
8) Check the quality of your brushes
Make certain that you have some HIGH quality brushes. Just for the detail work. I have since I started acquired 15 brushes of different quality to be able to do all the parts of the tiny models. Fine brushes are most important when it comes to quality.
9) Set Goals
Try to have a goal and complete it in good time. I set a goal for myself this week to get all the basecoats and black detail for my Dire Avengers completed. It is now Wednesday and I am about half way through. This gives me measured achievement and helps me to stay encouraged while painting.
10) Remember which colors you used
I started off with a Dire Avenger squad, and recently got around to adding another squad to my army. Problem is I don't remember all the colors I used to paint the original guys. I have to identify the colors from the model now. So if you paint a model, list the colors used somewhere to refer back to.
I will add to the guide as a learn. Hope this is helpful to someone.