First of all, I'm addressing someone who may have an old guitar and is looking to upgrade. If you are a first-time buyer, the best advice I can give you is to stay with one of the recognized names like Epiphone, Recording King, Silver Creek, Ibanez, Alvarez, Cort or Yamaha. All these companies make some very nice entry level instruments. Count on spending at least $200 for something decent, plus the cost of a decent gig bag or case. The big online retailers offer dozens of choices.
But let's say you have an old guitar that has not seen a lot of use over the years, or perhaps has been a faithful companion but you want to try something different. The first consideration is size. For decades the most common size of acoustics was the dreadnaught, or jumbo size instrument. This still holds true and the reason is no doubt that even in a fairly inexpensive instrument you can be sure a dread will give you plenty of volume, which is something most people want and like. Unfortunately, many people - women, kids, adults who are not large people - often find the dread unwieldy and difficult to hold. Women in particular often have trouble adjusting one of their...ahem...physical features to comfortably hold and play a dread. The guitar makers are more aware of this these days and a better choice if you fall into one of the groups I mentioned is the 000, OM or "grand concert" size instruments. These guitars are smaller than dreads but still offer plenty of volume and depending on the overall quality of the instrument, quite pleasing tonality for either light/moderate strumming or finger style playing.
There are also even smaller instruments (00, concert, parlor size) that are quite comfortable to hold but you cannot expect to get much volume or resonance from them unless you're prepared to spend thousands on a boutique or upper end American guitar.
I'm also assuming you intend to buy a guitar with steel strings. Unless you plan to play classical guitar or perhaps Brazilian jazz, nylon string guitars do not offer the variation in tone and most importantly, the volume of a steel string guitar. Sometimes people will want a nylon string guitar because the strings are somewhat easier to press down but that advantage is negated by the fact that on almost all nylon string guitars the neck is very wide, meaning you will have to reach farther around the neck. Occasionally people ask if they can put nylon strings on a guitar made for steel strings. The short answer is yes but because steel string guitars are braced for the the much higher tension steels, you will not get anything close to the volume or overall sound the guitar is capable of with the nylons, which cannot vibrate the top enough. So stick with steel - yes, they hurt a bit in the beginning but I promise you will be much, much happier in the long run.
Next choice is materials. I've written about the tonal qualities of different woods in the space before so I won't go into that now except to say that rosewood tends to be more resonant than mahogany (the two most common body types) but mahogany tends to have more "even" tonal qualities with (usually!) a better, cleaner treble end. The most important consideration is a solid wood top, which is usually some sort of spruce. Composites - a fancy word for plywood! - is common on inexpensive guitars' backs and sides. All solid wood is better but the price will reflect this. Fortunately, as opposed to a decade or so ago, even inexpensive guitars today have solid tops. I would definitely advise against buying any guitar that didn't have one.
Next I'll talk about features and their effect on the sound and "playability" of the guitar. Until then....
Peace & good music,