Picking a Warhammer 40k army is very much a personal choice which depends on a number of factors. In my opinion, these are the things you should be taking into account:
What's your gaming style preference? - Do you like getting up close and personal with your enemy, or would you prefer to hit them hard from range, or a combination of both?
What kind of army do you want to play? - Now, this question has several aspects to it. There's the "touchy feely" stuff - do you want to play humans or aliens? Good guys or bad guys, or the folks that just don't give a damn and want to smash everything?! Reading the background of each army will help you to decide on this. There is, also, the actual look of the army. This is pretty important, as you may be spending quite some time modeling and painting your army, so you've gotta pick one that you find visually appealing.
So, you've got a preference for infantry. In my opinion (and I don't mind others saying I'm wrong on this!), I think this counts out a few available armies, so I'll cover them last. In terms of infantry, you can either go 'elite' or 'horde'. Elite armies are going to be smaller but have really tough troops, whilst horde forces rely on overwhelming your opponent. Here are some I think may be good choices for you:
"Potential" Horde Infantry Armies:
Imperial Guard - These guys are your basic human grunts, and they don't look too far removed from a modern 21st Century army in many respects. They've got pretty weak troops in the main, but you can field loads of them! You can go for a very troop heavy army, or one that fields loads and loads of tanks, or a combo of both. They're pretty versatile in that respect. In terms of background, like other Imperium forces, they're deemed as being 'good guys'.
Orks - The classic horde army. Even in a small points game, you can field absolutely loads of Ork Boyz, which in the game is know as the 'Green tide' - of course, your basic Ork isn't hugely impressive, but they rely on sheer number of attacks thanks to the large number you can take. Whilst the Imperial Guard can hit hard at range, Orks are not renowned for their shooting ability! Instead, you're going to have to get close to your enemy to truly make the most of these guys. In terms of background, I kinda think of these guys as being 'neutral' rather than bad guys. They just like a good fight, and they don't care who's skulls they're bashing! From a modelling perspective, you have quite a lot of freedom with Orks - they're an anarchic bunch, and all their equipment is cobbled together - or 'Kustom', as it's known in Ork-speak, so you let your imagination run wild!
Tyranids - Think horrific aliens looking to consume all before them! These guys can be fielding as a horde army, and have some nasty units. If you're thinking of playing them, I'd wait until January 2010 though, as they have a new codex coming out then. At the moment, there are some problems with the existing codex and 5th Edition rules, so they're not 100% at this current moment in time. I personally don't think they have as much character as some other armies, but this is my own opinion. They are a popular army, and look pretty impressive on the table.
Necrons - Now, these guys can be seen to be a bit more elite than the above choices in some respects, but I personally believe them to be a horde-esque force. Sure, you can't field as many of them as Orks, but any Necron army really really needs to be fielding lots of Necron warriors to get around the 'Phase Out' rule. These guys are the unemotional, unmerciful robots of the 40k world - think the Terminator films! Now, many players think they're a bit dull and predictable to play, but it kinda depends on whether you like robots / androids or not! Unfortunately, I don't think they will offer you as much fun in terms of painting and modeling as some other armies.
Elite Infantry Armies:
Space Marines (Codex: Space Marines & Specialist Chapters) - Space Marines are hard as nails. They're the elite super soldiers of the Imperium. This means you can't field as many of these guys as, say Ork Boyz or Imperial Guardsmen. However, the average tactical marine is a great unit, and very versatile, so you'll want to field as many as you can! The standard Codex: SM army is balanced - you've got some good tank choices, as well as the great Space Marines themselves. In terms of painting, you can pick an existing chapter like the Ultramarines or Salamanders, or even create your own chapter with a colour scheme of your own creation. If you don't know, a 'Chapter' is the term for a particular Space Marine army, like a regiment basically.
There's also the specialist chapters - these include Black Templars, Blood Angels, Dark Angels, and Space Wolves. Whilst they're all essentially Space Marines, and share similar units, these chapters have their own Codexes which give them their own units and special rules. In the whole, they're more spe