I am a huge newbie when it comes to Warhammer 40k. I'm starting to get into the game, learning the lore and such. I'm hoping to get an army started and would enjoy and appreciate some input on what I should choose.
I really like three armies, the Tau Empire, Imperial Guard, and the Space Marines.
Welcome and congrats on discovering 40k. It's a cool game. People say it's expensive, but once you've covered the initial outlay it's actually cheaper than most hobbies (certainly cheaper than xbox/PC gaming).
Basically, the answer to your question is Space Marines. If you are starting out in 40k, they are the cheapest, easiest, most diverse, most popular, most powerful and probably the most interesting army (certainly, they have the most lore written about them). I love Tau and IG, but as a 'getting started' army, Marines can't be beat. I would recommend buying 'Dark Vengeance' (the 6th edition boxed set, which comes with two armies for $85), selling your Chaos army on eBay for $40, then topping up the basic army that comes with the boxed set (which comes with everything you need to play- huge bonus). Basically you've got yourself set up for games at only $45- (about the price of a single xbox game).
My next piece of advice is find your local gaming club. If you're gonna play, you'll need opponents. I've never come across a gaming club that wasn't welcoming and keen for new players- there is bound to be one in your area. Not only will they give you army-gathering advice, they usually have people selling models very cheaply (especially Marines, which are abundant) and will probably let you playtest a few armies to see which you like. If Marines aren't to your taste, sell `em on eBay and switch to the next one. eBay is your friend when it comes to cheap armies (GW will gouge you on price- badly. Always look for the alternative before buying from GW).
I don't know if you're a painter but it pays to learn. It's a great part of the hobby and even if you don't have an artistic side you might surprise yourself when you start painting- I certainly did.