There are some great answers in this thread already. I'd echo those who've said it's important to figure out your motivations for playing the guitar and determine what kind of music you hope to play. Learning how to properly tune your own guitar is a great tip as well.
For my part, I'd recommend learning to read music as soon as you can. Guitarists often have the option of reading tablature instead of standard notation. Tablature (or "tab") uses a six-line staff, one line for each string of the guitar. Numbers are printed on the lines to represent frets of the guitar. For example, a "6" printed on the third line (counting from the top down) tells the guitarist to place her finger on the sixth fret of the third string.
On the other hand, you have standard notation, which is the form of written music most people are familiar with. It represents notes using closed and open circles arranged on a five-note staff.
There's nothing wrong with reading tabs. I'd still recommend devoting time to learning standard notation at the beginning, though. The concepts behind it are simple enough, but it can take years of practice to really get fluent with it. As you advance, you'll likely want to work through guitar books that don't have any tablature at all. So, why not start the process of learning to read music right from the get-go?