For some people, it's pretty simple ; either they already know what they want to play/paint, or they want the flavor of the month/strongest army (right now, i'd say Nurgle Warrior of Chaos)
For the others, well, warhammer fantasy has a total of 15 armies, all very different from one another, and each have their strenght and weakness.
The first thing you'll need to consider, is what kind of army you want, or more precisely, what do you want from your army? there's many right answer to this.
Some people want an army with few models, like Ogres Kingdoms.
Some people want an army with lots of monsters, like Daemons of Chaos.
Some people want an army that is simple to paint, like the Lizardman.
With you, i'll review each army, to hopefully help you decide which army is right for you.
In no particular order (well, alphabetically) ;
Beastmen by Abraham Danglebreech
For the painter, Beastmen are a bit boring. Their model usually are pretty brownish, and their lord/Heroes choice are pretty outdated. Some people manage to do amazing things with them (http://www.warseer.com/forums/showthread.php?277226-Infrequently-updated-Mutant-army-project), but for the most part, i personally think they are not really of interest paint wise.
For the gamer, Beastmen are horrible. To give you an idea; most tournament give beastmen extra points to overcome their lack of competitiveness. Sometimes up to 20%. And still, pretty much nobody plays them.
Knights by Eclipseddstny
For the painter, Bretonnia is pretty interesting ; you can go pretty much any color you want with both cavalry and infantry. They usually come in bright color, which usually is very nice looking on the table. Obviously, you need to like painting horse. Personally i have a love/hate relation with Horse, so i wouldn't pick them, but for someone who likes Knights, it's a very nice army. Even with older model, most of their range is still very good looking.
For the gamer, Bretonnia is below average. With steadfast in 8th edition, cavalry have alot of trouble dealing with very deep units. they will win combat, but they will most likely never break the ennemy, and after the charge, cavalry is alot less frightening.
Daemon of Chaos
Yours truely Nurgle army
For the painter, Daemon of Chaos offer alot of different options, since all four chaos gods are completely different. Nurgle offer the "lets make something out of this ball of green stuff", and you can pretty much get away with anything if you say its nurgle. Khorne is the master of red, skulls, and fire ; sculpting flames, painting red, all is good. Slaanesh is about pleasure, naked ladies and purple. Tzeentch is about magic, pink/blue and weird looking monsters. And since you can mix and match, its really the best army for someone with short attention span.
For the gamer, Daemon of Chaos is a top tier army, which means you'll most likely end up winning games. The different god each offer different game style, so you can tailor your army to your play style, no matter what it is. You want to be a combat monster? Khorne. You want to slowly grind your opponent to pulp? Nurgle. You want to magic your opponent out of the table? Tzeentch. You just want a bunch of crazy lady rushing head first into the ennemy? Slaanesh. And no matter what, pick 2 Skull Cannon!
Yours truly, once again
For the painter, Dark Elves are pretty nice; most model are new and in plastic, so they are easy to assemble, to paint, and to convert. They are somewhat in the middle between an army of many (like Skavens or Goblins) and a least-model-possible army (like Ogres), since most of their model are very expensive points wise. The good thing about that army is that depending if you like to paint armor or skin more, you have a perfect choice for you.
For the gamer, once again, Dark Elves are near the top of the food chain. They are not the "best" army, but they definately are above most.
Dwarf by GriffinPainting
Note ; Dwarf are due to be release next month (february 2014), so this review could be completely false by the time you read this
For the painter, Dwarves are... boring? There isnt much except infantry, from well armed tin-can to completely naked, and warmachines. On top of that, their model are outdated, and somewhat lack any feeling of movement.
For the gamer, its what come closer to 40k ; You stay back, and nuke the hell out of your opponent. Kind of boring for those playing and facing the army.
High Elves by Lan Studio
For the painter, high elves are like Dark elves (duh!), only lighter. Still an elite army with few models, and despite some of them a bit outdated, the army is still stunning.
For the gamer, high elves are above the average army, and could definately fight with the top one. As an elite army (and low toughness), it need careful playing compared to some other point-and-click army.
Lizardmen by StrategicCommand
For the painter, Lizardmen are a blast ; scaled skin is one of the easiest thing to paint, and lizard come in all color, so you are really free to paint them as you want. Pink? no problem. Black? no problem. Their monster are very nicely done, and on the bright side, they even got a new book !
For the gamer, Lizardmen are a beast in CC, as long as your opponent isnt Purple Sunning you to oblivion (or whatever paradise lizard goes to). They got great shooting with skinks and salamander, and great CC with saurus. The Slann used to be the mother of all caster, but now he's tone down to just a great character overall.
Part two, to come soon.