Well, technically you already have an "acoustic" guitar, but I've been around enough to know you're looking for a steel string acoustic.
There are a few things to consider, what style of music are you going to play mostly, and how are you going to use the guitar; ie: play in front of people, strumming around a campfire, leading church groups, or playing on your couch by yourself and do you need electronics?
Being the guitar slob I am, I always say buy the best guitar you can afford (then spend just a little bit more). It's a lesson you usually have to learn yourself once but if you buy a cheap guitar and continuing playing you will quickly outgrow the guitar and have to spend all that money again plus more to get a better guitar.
So what makes a good guitar? Whatever sounds good to you. That's the bottom line. The catch is that when you're just starting out you likely can't hear a real difference between a guitar that costs $300 and one that costs $3000 (honestly it may not be there but we'll assume). That's where having someone who has played before help you comes in very handy. And make sure you try more than one of a guitar. Guitars are all unique, they will all sound different (even the same model), some are gems other are not. I could go on and on about purchasing guitars but I'm going to head to bed.
My recommended "budget" guitars would be Seagull S series, Martin 15 series, Larrivee 03 series, and the Breedlove Atlas series.
Try browsing these boards:
http://acapella.harmony-central.com/forums/http://www.acousticguitarforum.com/forums/index.phpDon't be afraid to go for a used guitar, you can get a great deal and the guitar will probably sound better since it has been broken in.