Simply choosing an army in this game can be quite annoying. We all have found that someone else has a better army than ours, or we just get tired of doing the same thing over and over. The best advice is to plan and prepare. Three of the greatest things you can do in choosing an army are: reading many codices, general research, and watching people play their armies. I have gone through 4 armies now. I have happily rested on the Sisters of Battle.
When you think that your army has become stale, or you need a new one, you should ask people to borrow their codex and take a look through it. Sometimes you may be convinced of the army because of one great character (Kayvan Shrike the only reason I bough Raven Guard) or because they have great abilities (Daemonhunters, Sisters of battle). You may also want to check if GW has released any add-on lists like Kult of Speed or Armored Company. The most common army I have seen played or picked up is Eldar for the sheer L337ness of the individual units. No matter your preferences the Codex is the best way to know what the strengths and weaknesses of the army are.
After you have picked a couple of armies that look really interesting you want to know what others have to say about the army. One great way is to go to a forum. People will give you great advice on lists, tactics and rules that you need clarification on. Once you have learned about the armies that you may want to pursue make a chart and decide on the strengths and weaknesses. Pick you favorite army out of the group. You are now ready to head in to your local game store.
You have already decided on an army, but you want to see how it works for others. You are at the local store and you notice that someone is playing the army that you want, go over and see how he is doing. If you have been playing for a while then you should know the good generals in the area, if not then don't worry you can still get some good info. After the game ask him (or her) what they think the strengths of the army are, why they play that specific army, and would you suggest it to others. After that ask them how easy it is to play (this is a good factor to know if you are new, the vets should be pretty comfortable playing just about any army). You have either decided that this army is a good or a bad choice.
If you learned it was what you were looking for then you will soon be the proud owner of a new army and you will have spent $300+ to get it. If you still need to look for the right one then go back through the steps and know that you saved $300+ for a later date but will still be out $300+ dollars. You now have the knowledge to go and use the new army effectively because you have learned both the codex and tactics that have worked for other generals. If you just can find the right army for you then look at Kroot or you may want to check if GW has released any add-on lists like Kult of Speed or Armored Company. Happy gaming and enjoy your week.