So you're looking to buy a new guitar and you don't know anything about guitars. Here's some advice:
Styles
Guitars come in many shapes and sizes too numerous to name here. But so you have the general categorization:
Steel string acoustic -- Most guitar students buy steel-string acoustic guitars because they are so versatile. These guitars sound great for rock, blues, folk, country, jazz, and everything in between.
Nylon string acoustic -- Sometimes known as a classical guitar, these guitars have nylon strings so they sound quite different. Typically these are used for classical guitar, flamenco, and many latin styles of music. Of course, they can also sound great for folk strumming, and many students do start out with this type of guitar.
Electric guitars -- If you wanna rock out, this is what you'll need eventually. You'll also need to buy an amplifier and cables to go with it.
Brands and Price
With guitars, spending a little extra money can make a huge difference. While there are many brands of guitars these days that are fairly well made, even at the $300-$500 range, if you spend a little extra money you will get a guitar that is much easier to play and better sounding.
If you think about it, that guitar will last you your entire life, unless you throw it down the stairs. So it's a good investment. My favourite brands are Larrivee, Martin, and Taylor for guitars that are in the $800-$1500 range. Larivee is a Canadian company, which is kinda cool. I play a Larivee D-03R.
However, if you are on budget, the brands I recommend in the lower price range are Yamaha, Seagull, and Simon and Patrick. You could always upgrade to a better guitar later. Guitars are generally easy to sell or trade in.
Size Matters
Make sure the guitar is comfortable to hold -- not too big to get your arms around especially if you are a smaller person. The Dreadnought guitar is the most popular shape for steel string acoustics because their large bodies give great response in both the bass and high frequencies. But they are rather big and if you're on the petite side they may be uncomfortable to play.
Action (string height)
One thing to seek out is lower "action" -- this means that the strings are not too high above the fretboard so that you don't have to push too hard to fret the notes. Action is usually measured at the 12th fret. Compare how high the strings are at the 12-fret on different guitars.
Some cheap guitars have high action, which means it will be harder on your fingers, especially for a beginner. Many stores will do a free "setup" of a guitar after you buy it, which could include lowering the action.
Solid Wood or Plywood
For acoustic guitars, companies spend a lot of money marketing solid wood tops versus plywood. Guitars with solid wood tops tend to sound better, however, they are more expensive. If your budget is limited, don't be too concerned about this. The difference is minimal, especially to a beginner's ears. It's more important to get a guitar that is comfortable to play.
Looks matter
It's important to buy a guitar you're attracted to. Buy a guitar that will inspire you to play it often. Get a guitar that speaks to you as you spot it from the corner of your eye and it says to you: "Pick me! Play beautiful music with me!" Let's be honest, part of playing guitar is looking cool, right?
Second-Hand Guitars
If you're considering buying a second-hand guitar, all of the above advice applies, but you should also check that the neck of the guitar is not warped.
You can do this just by looking straight down the neck from the body of the guitar looking lengthwise, straight down towards the head stock.
The neck can be slightly bowed, which is fine, but it should be either straight or very close to straight so that the height of the strings above the fretboard is fairly consistent over the entire neck. If it's excessively bowed, do not buy the guitar. It will be difficult to play and this is not something that can be easily fixed.