capricorn one is right--Johnny Cash was basically just a rhythm player.
Anyway, I'd recommend starting off by learning simple chords such as C, F, G, A, Em, E, D, Bm and Am. With just three of those chords--D, G and A, for example--you can make up tons of different chord progressions with many different melodies. Anyway, there's also a great finger exercise in which, starting on the lowest string (low E) with your index finger, you play the first fret. On the second fret of the low E string, you play with your middle finger. In the third fret, your ring finger then, on the fourth fret, use your pinkie. Then, you do the same thing on the next lowest string, the A string. Work your way from the low E string up to the high E string, then do the same thing backwards. This simple exercise will help in building up your fingers' dexterity and strength. Also, make sure to play along to a metronome. This way, you'll develop a sense of rhythm. Actually, I'd recommend starting with that finger exercise instead of chords. Then, when you get that finger exercise down, move onto simple chords.
Johnny Cash's "Folsom Prison Blues," if I remember right, used three simple chords: G, C and D (maybe a D7 instead of a D--I can't remember for sure). Basically, the chord progressions in his songs were always very simple like that. Of course, learning more complex chords such as A13, D9 and E7#9 would always be great to do...after learning simpler chords first, of course. Also, learning scales is a great thing to do eventually, starting with the minor pentatonics (the most often used scale--used not only in country, but also in rock, jazz, blues, etc.). Practicing scales, like practicing that finger exercise, can also help build up finger dexterity and strength. Just makes sure that, when you practice, to always use a metronome (or at least a steady continuous drum beat to keep a rhythm).
Just be sure to always keep one thing in mind...HAVE FUN! If you don't enjoy playing guitar, it'll just make it harder to learn. Honestly, that's why so many self-taught guitarists (such as session guitarist, Brent Mason) become such great musicians. They love to play so much that they play for hours and hours on end and, because of that, many self-taught guitarists can easily outplay those who have taken formal lessons.
Here are a few websites you can check out:
http://www.jguitar.com
http://www.justinguitar.com
I just thought I'd finish with a list of some great guitarists, from different genres, to check out. Listen to these great musicians for inspiration to learn as much as you can:
Brad Paisley (solo artist; country--he can play ANY style with ease)
Brent Mason (session guitarist; country--he can play a lot more than just country though)
Danny Gatton (solo artist; when alive, Gatton played everything including rock, blues, jazz, country and rockabilly)
Jerry Reed (solo artist; country...he was a great finger picker)
Chet Atkins (solo artist; country...another great finger picker!)
Pete Ham (Badfinger, solo artist; rock...one of my favorites)
Django Reinhardt (solo artist; gypsy jazz...he only used two fingers)
Eric Johnson (solo artist; rock)
Melvin Taylor (solo artist; blues)
Tom Keifer (Cinderella; hard rock...kept it simple, yet was a great songwriter and an all-around great musician)
Redd Volkaert (solo artist, Merle Haggard; country...he can also play rock, blues and jazz quite well)
Cody Kilby (Ricky Skaggs & Kentucky Thunder, solo artist; bluegrass...one of the fastest pickers I've ever heard!)
Johnny Hiland (solo artist; country, blues, jazz, rock...he's great and, get this--he's BLIND!)
Steve Vai (solo artist, David Lee Roth, Whitesnake; metal...technically, one of the best guitarists out there)
Wes Montgomery (solo artist; jazz)
Mike Stern (solo artist; jazz)
Sonny Landreth (solo artist; rock, blues...the best slide guitarist I've ever heard!)
Monte Montgomery (solo artist; acoustic...brings a new meaning to "acoustic")
Hope this helped!