(I'll try to leave this somewhat generic to apply to any system)
If you just want to win, Google yourself a bunch of tournament info and find out what lists the winners are playing, then copy them. Does this mean you'll always win? No, but it might give you a leg up if you can apply the same tactics against your local opponents. Do I suggest this method? Not in the least.
Now that that's out of the way, here's my suggestion - research the armies: read their backgrounds and look at pictures. Something in there must have sounded and/or looked cool to you - a part of fluff that piqued your interest. Read more on the army(ies) that stood out to you and look at more pictures. You're going to invest a lot of money and time into this game; you'd better like what you choose.
Once you've found an army that appeals to you, figure out if you can live with that decision. Don't get me wrong, I still think number 1 is pick what interests your brain, but all armies play very different. If you're not going to enjoy a certain army style, you might want to look at something else. All armies will generally have both pros and cons, and if you can enjoy playing with your cons, go for it.
Here are some of the pros and cons you may find while researching armies:
- Do you feel like standing back and shooting, not moving your troops around much? Dealing out lots of damage at long range, but folding at closer range?
- Would you prefer a smaller army of stronger and tougher troops, thereby giving your opponent less things to kill before you lose?
- Do you want to bring a knife to a gun fight? (or vice versa)
- Want to play the war of attrition and play the odds? Lots of numbers usually means you need them all to be effective and they'll die easily.
- Prefer to be slow and steady to defeat the hare?
Everyone knows Space Marines, bro, are totally Games Workshop's golden children... bro, especially those blue Smurfs, bro. Not to be outdone, yes I play an army of dandy space elves prancing around the galaxy hiding from anything scary.
There's nothing that says you can't change your army at some point down the road. (Except perhaps a financially minded spouse) This is an expensive hobby, especially if you like having a side-bar of options and like changing your army list around. It won't be "Am I allowed to change my army?" It'll be "Can I afford to change my army?"
Before making the leap, consider whether your current army can be fielded in a different manner - perhaps you'd have more fun playing them differently. Nids and Orcs can do non-swarmy and Eldar don't have to rely on vehicles. Nids even have viable shooty army options. Try it first, and see if you enjoy your army more.
When you're deciding on a new army, you first need to figure out what aspect of your current army you want to change. If you're playing an all "shooty" army, don't pick another race that does the same. Microing 100 models as a nid player won't be much different as an orc player. If you're investing in the change, try something different. Maybe you just want a different "look" or have a vision of an army you'd like to paint.
If you want to play around with other armies, you can pick up a codex and play a bunch of games representing all those models you don't have, and see if it'll float your boat. If you're contemplating a swarm army and don't have enough models, then (assuming your friends are fine with it) go and buy a bunch of cheap army men. Don't get used to playing this way, because your friends will probably expect you to make up your mind and commit. (see: spend a lot of money on new models)
I don't recommend selling off your old army. You might not like the new army, or feel nostalgic and want to play the old one now and then. A spare army is also great for introducing new players to the game.
I play Eldar and I chose my army when I was just a kid. Looking back, I'm glad I made that decision. I've thought and been questioned from time to time as to "Why Space Elves?" and recently I came up with an answer that makes a lot of sense to me - they're completely science fiction. I love their look, their sleek form and alien technology. While a fan of the cyber punk and post-apocalyptic settings, 40k is neither to me. I see 40k as Science Fiction and the Eldar ARE Sci-Fi.
Image-wise, this is my big generalization of 40k: (emphasis on "my personal view")
To me, the Imperials are for the classic war nut and the Marines for the modern war nut. Chaos allows you to be the destroyer of worlds and Space Orks bring fantasy to 40k. Nids create a gritty Gigeresque alien, Necrons for the Terminator fans and Tau added the mecha flavour.
Here's the homework assignment: (doesn't matter what system you play)
How and Why did you pick your Army? Did you ever Try or Change to another army later, Why?