Let's get down to the actual decision making process now. We've looked at the 3 aspects of 40k; the tabletop, collecting miniatures and Black Library. For choosing a faction, I recommend blending all 3 aspects together.
Your first step should be to just look at some miniatures. Find a local Games Workshop store or an independent retailer (both can be found using the store locator on GW's website). Go to the store and just hang out. Look out some completed minis and watch some battles. Ask questions and learn what you can. Many store managers will also suggest ways to get into 40k. Check out some of the boxed kits and see if anything jumps out at you (the price probably will unfortunately). You might find your interest already captured by a particular army, and you haven't spent any money yet. Watching battles is a good way to gauge your interest as well. Go to the GW website and just scroll through their 40k section. The more exposure you can get to the game the better. And whatever you do, resist the urge to buy stuff, especially from high pressure store owners. Eventually you will have to spend a fair amount of money, but avoid impulse buying, which will only lead to wasted money and discouragement.
During this initial phase, I would recommend buying some Black Library books to fuel your interest. A single book will generally only cost $8-10. Your goal here is to understand the lore and background of some of the factions within the game. I read through the Salamanders book series when I was first starting out and now I plan on doing a Salamander army soon. Fielding an army with a back story you like will make the game more enjoyable.
Hopefully by now you are finding an army that interests you whether through lore or looks. In my opinion, these two aspects are the most important. If you choose an army simply because of its fighting power, you will most likely end up with a bunch of unpainted minis (and we have more than enough boring gray plastic armies to go around).
Getting a codex is the next step; however, a codex generally costs around $60, but a digital version for an iPad will be slightly cheaper. Before you rush off and buy a codex, take some time to read and look through some store copies of a codex whether at GW or an independent retailer. A codex will have a great deal of lore and showcase many of the faction's heroes as well as the fighting strengths, weaknesses and styles. Don't buy a codex until you are committed to that particular faction.
Once you have found a faction with a back story you like and that looks good to you with fighting characteristics that you want, start searching for a paint scheme. For beginners, choose a scheme you like by all means, but keep in mind the difficulty of your scheme. I got burned out when I started 40k because I was building a Salamander force with too complicated of a scheme, including mixing paints and highlighting along with almost 5 or 6 coats of paint per mini. Too much work for someone just starting out. Genesis on the other hand is relatively simple, no mixing paints or highlighting, and only 3 coats of paint.
To sum it all up, find a faction that appeals to you through its lore, the look of the minis, its fighting characteristics and then choose a stylish but uncomplicated paint scheme. Go for originality, choose a faction that isn't fielded often with unique paint schemes, and build fluffy, lore based battle lines. And above all, never play with unpainted minis!