While I am in favor of a teacher to guide you as mentioned in the first two answers I would like to expand on all of this.
The first thing is getting a guitar. If you already have one, have your teacher check it out. Also, have your teacher show you how to care for the instrument, and how it should be set up, as well as going to a good luthier for a set up.
I would recommend a nylon string acoustic guitar for starters. When I was 12 years old
I wanted an electric more than anything but when Christmas arrived that year, I was fortunate to be given a nylon string acoustic. Although I was a bit disappointed I quickly learned that having a nylon string guitar was better for me as a beginner as it was so much easier on my fingers before I developed calluses--a steel string would have made this a lot harder to learn.
I had already started piano when I was 10 so having 2 years of piano and basic music theory helped me start learning scales, chords, and arpeggios from the start. It wasn't until a year or so later that I took a few lessons. If I was headed for classical or jazz guitar I would have continued the lessons, but soon I was in a rock band and learning pop songs which was what I wanted. I encourage you to hook up with like minded folks and play music together even if it does not lead to gigs. Working with others is a great way to learn to play live, on time, and make music. I eventually moved over to electric bass and played in a blues band during my high school years. This was a great experience to learn how to be a part of a rhythm section and how to support the beat.
By this time I had acquired a couple of electric guitars and was still learning on my own.
I eventually studied composition in college and graduate school. I have always had a guitar handy to compose with as well as access to a piano which for composing are very helpful tools.
Many years later, I returned to blues and got a stratocaster and a vintage fender black face amp and still play these with a small collection of other electrics and amps too.
I jam with pals but rarely play gigs, but love playing. I always find new things to learn so I mention this, as guitar like anything artful has no end to learning and expressing.
My focus is composition and making music for my own films so my development has a lot to do with the music that I think works on my current video project.
Get a good starter guitar that will facilitate learning.
Get lessons from a teacher that can guide you to the music you want most to learn.
Work in an ensemble or combo to get use to playing with others.
Continue your development as your passion drives you, take music theory in college or high school or if you're beyond that age, take night or weekend classes at your local junior college or continuing adult education center.
Never give up, keep working on the things that you want to master, and you can always find a teacher if you get stuck.
And finally, be willing to take chances, don't be afraid to learn things that seem impossible now.