Just dive in man, you'll swim. You definately could start off learning little kiddie songs like Marry had a Little Lamb: I did, I worked all the way through the Guitar Method books that SoS talked about. They helped me in two ways:
1) It gave me the necessary basics of playing the guitar such as holding it and strumming and picking (now that I look back, those songs make great alternate picking excercises).

2) It gave me the drive to learn stuff on my own. So while I was learning Simple Gifts, I was also learning a powerchord version of "Wild Thing", which at the time I thought was so cool.

However I do recommend you get a teacher. It is very important to develop good habits and not bad ones. Remember, it takes 7 repetitions to form a habit, and 70 to correct it. I think... where did I hear that.. anyways. Get a teacher!