First off, buy a six-string acoustic guitar (not a seven or twelve, but a six-string one) at any guitar store I personally shop at the "Guitar Center" since they offer the biggest selections of quality guitar, bass, keyboard, drum, and audio equipment at bargain prices. Also, I believe they provide very good customer service and satisfaction overall. So if you ever have a chance, check it out and bring a friend along. However, if all those pricey $100-$3,000 Fender and Martin guitars intimidate you, you can always go to a pawnshop or look for a used one that no one seems to use around the house. Big-named brand or not, as long as the guitar has six workable strings, it should work fine. But if you are as picky as I am about the sound quality or feel of the strings you should compare different guitars with different prices and go from there. In addition, try to get into the habit of asking as many questions as you can think of to guitarists you know and guitar salesmen. It might bug the hell out of them, but in the long-run it will help you to become more guitar savvy. And who knows? After you are done asking those questions, you could actually end up saving a few bucks because you bought what you needed and rejected what you did not feel was essential. Personally, I once knew someone who went all out on his first guitar spending spree and bought himself an electric guitar with an amp, a monster cable, thirty sets of extra strings, a footswitch, et al. but ended up wasting a lot of it because 1) he quit playing and, therefore, stopped using $800 worth of equipment and 2) he never asked anyone else for advice about what he needed to start off with. He realized later that he probably should have started out with an acoustic and would have only spent $35 had he asked just a guitarist or two for insight and their opinions.
As far as what you should know beforehand, be aware that there are basically two different types of acoustic guitars: the half-size guitar and the full-size shape. The half-size ones are cheaper, smaller, and intended for 5-10 year olds while full-size adult models are larger, generally more expensive, and designed for anybody who is about 4’ 9” or taller and /or older. If you are about 4' 3," regardless of age, then go ahead and buy the small one. Your arms will be too short for the full size model (no offense). On the other hand, if you are at least 4' 10," then you can look into buying the adult conventional sizes, which are the ones that most people play. Though some adults choose the smaller guitars, I highly recommend the full-size models. The reason being is that larger guitars have bigger sound chambers, which means they tend to emit better tones, vibrations, and overall harmonics than the half-size ones. It is like the difference between a cassette and a CD; most choose the latter since it has better sound quality, even if it is a little bit more expensive. So if you can, spend those extra fifteen or twenty dollars. You will thank yourself later for doing so.
One last thing that should be discussed when looking for your first acoustic guitar is the strings, which come in either nylon or steel. Putting aside the gauge (thickness) or brand name, nylon strings are easier on the fingers while steel ones will dig a little bit deeper into them and create bigger and rougher calices. When I choose guitars, I usually pick ones that come with steel strings. The sound quality is a whole lot better when you play with them and your fingers do not clumsily slip along the frets as often. Therefore, I usually do not care too much for nylon. The logic here is, who cares if my calices are 10% deeper? If I did, I would have said, “screw this!” the first time I touched a guitar and would have never seen the talent that was inside of me.