First of all, welcome to the hobby. It's a great way to meet new friends, and much cooler then videogames. Also get ready to have no moneys.
As for armies, I can only talk of my own experience with my Orks, Imperial Guard and Chaos Demons.
Orks are my favorite army to play, as having a huge horde of green advancing towards the enemy supported by colourfull and wacky vehicles is just... awesome. Stay clear of too many specialist squads, tho, you'll have most fun with a huge Boy and Gretchin horde. All those dice in your hand can realy freak people out.
Imperial Guard is my strongest army. A huge wall of infantry with an ungodly amount of tanks, wich are the best in the game, by the way, is hard to defeat by less competent generals. It sounds like an Ork army, but works very different. The standard infantry man is even more useless then an Ork Boy, allmost as bad as a grot, but supported by their officers, these lowly humans can best even the mightiests Space Marines.
Especially if you love tanks and being guaranteed to outshoot your oponent you should play these guys.
Chaos Demons are... a weird army. Highly specialised units that can only enter the battlefield by deep strike... that's just hard to work with. You can never realy get the right guys in the right place at the right time. The apeal from this army comes from the fact that your oponents are just as much in the dark as you are on what will happen next, and most important it's visually very diverse and stylish. If you like painting and modelling, this army is my personal favorite for that.
And affcorse, the old cliche still stands true: don't ask other people what you should play, but pick the army with the models you like most. If you are not sure on an army, you might regret it later and Warhammer regrets cost usually around 200 euros.