Foremost, keep in mind that codex creep exists. That basically means that your army choice might not be very powerful right now, but it will get its turn eventually being a "top tier" faction. So, although it's said quite frequently, it really is true to just choose the army whose aesthetics fit your style best.
Do not - I repeat - do not buy more than the basics to begin with. A rulebook (preferably the mini/pocket edition), a codex, an HQ and two troops. Why? Because although Games Workshop insists you need every single product they sell, this game is not "ready to play". With anything more than a couple boxes, you will overwhelm yourself and become so discouraged that you'll shelve the entire game. I've seen it happen.
Although going over the rules and your codex studiously and designing your army to be a synergistic force of devastation, most players seem to forget that their opponent is making an equally powerful, if not more powerful, army list. Expect to lose your first few games, even if it's against friendly/casual players. This game is all about trial and error, and luck. What steamrolled one army list might fail miserably against another.
Often, I hear people suffer from the intimidation of painting. I consider myself a decent painter. I'm certainly not a "dipper", but I am absolutely nowhere near Golden Daemon status either. But I will confess, when I first started years ago, I was a shadow of what my skills are today. Practice makes perfect.
And probably my least favorite part, the cost of the hobby. Yes, this game is expensive. I'm not one to side with those who try to defend GW's greedy price raises, but there are a couple things they get right: This isn't just a game, it is a hobby. And technically, all that is required are the rules and codex, and an HQ and two troops. All together, you're looking at about $200 to start.
Sounds pricy? It is, but let's compare to another hobby: Magic. The game requires a minimum of 60 cards in a deck. $10 starters typically contain only 30 cards. The cards themselves cycle out of play within a couple years and decks will need to be re-bought and re-built. Randomized booster packs and card rarities add more levels of poor matchmaking: Whoever has the deepest pockets will be able to afford the best cards. In 40K, everything is a level playing field. Yes, kits cost more or less than others, but nothing is rare or limited.
Now compare it to the #1 MMORPG: World of Warcraft. The game at launch in 2004 was $60, each of the five expansions were $40, and with a $15 per month subscription fee (multiplied by 10 years), the game has actually cost long-term players $2,060 so far.
With 40K, I've probably spent about... $2,500 in models, rules, glue, paints, and brushes since I started the hobby in 2006. That's roughly $228 per year, or $19 a month. That figure would actually be smaller had I not bought expansions or terrain pieces.
That's pretty much all I've got to pass on to any beginners or potential players. So now I leave you with this beautifully accurate cartoon by CTRL+ALT+DEL.