The first thing you'll need is pretty obvious: a guitar! You should really give some serious consideration to what you want in a guitar before you buy one, though.
There are three major classifications of guitars: acoustic, classical, and electric. Acoustic is pretty much the most versatile, but it may not be what you want.
An acoustic guitar is a steel stringed guitar with a hollow wooden body. As it's name suggests, it can be played acoustically, meaning that you don't need an amp to play it. You can use an amp with an acoustic guitar, though. Some come with pickups already installed inside the guitar - you just plug in a ¼" sound cord and go. These are called acoustic-electric guitars. If you choose a guitar without this feature, you can buy a soundhole pickup for your guitar for about $60, which allows you to plug your acoustic guitar into an amp.
A Classical guitar is similar to an acoustic guitar, except it has nylon strings and is designed somewhat differently. As it's name suggests, a classical guitar is ideal for playing classical guitar. You might also want a classical guitar for fnger-picking or Spanish music.
An electrical guitar is usually a plastic solid body guitar with thin steel strings. It requires an amp to play it. Electrical guitars have a unique sound, and are often used with effects pedals to distort or change the sound of the guitar. Electric guitars are central to rock music, as well as being integral to many other types of modern music.
Once you get your guitar, you'll still need a few other items. If you're playing an acoustic or electric guitar, you'll want some picks. I'd suggest you start with Medium picks for most music. You should also buy a capo, which is used for changing the key of your guitar. Be sure that it fits your guitar properly before you buy it though.
With these items, you should be ready for your first guitar lesson.
