First, you need to beg, buy or borrow a guitar. A basic nylon-stringed guitar is ideal- you don't need a really good guitar to start with (you'll probably bump it a few times until you get used to handling it, anyhow.)
Don't be scared to get a cheap second-hand guitar in good condition; guitars can mellow with age. Check that the neck is straight, that the strings are not too high off the neck, that there are no splits in the wood, and that it has a nice tone when you strum the strings. It will probably need new strings, so check the cost of re-stringing at a music store and factor that into the total cost (later on, you can re-string it yourself, but you may not want to do it yet if you're not familiar with guitars). If you have an experienced guitarist friend, they may be very helpful with choosing and stringing a second-hand guitar.
You'll also need to have some way of tuning your guitar- a keyboard or (in tune) piano is fine if you have one, but it's better to buy an electronic tuner for guitar- it's easy and accurate, and they are now quite cheap as they have become mass produced.
You can also get a free online tuner at get-tuned.com, HERE (you need to click on each note, listen to the sound and then tune your string to the same sound), or you can download a free guitar tuner app for your computer or smartphone; some apps will even use your microphone to tell you if you're in tune.
A capo is also very useful so that you can make the pitch of the guitar a bit higher to suit your voice when you sing along (see below for different types).
That's all you need to start playing, once you have some simple songs, chord diagrams and some basic instructions... you're ready to play your first song (see First Guitar -easy songs)!