Watch the Tuning the Guitar online guitar lesson by Rich Maloof from Acoustic Guitar For Beginners

An electronic guitar tuner is a sound investment in your playing and in your sanity. You'll develop a good ear in time, so that you can tell when you're in or out of tune, but for a few bucks you can take out all the guess work.
Also get yourself some thin picks (flat picks), and a strap if you want to play standing up. You'll also want a metronome or some other way of keeping time. The Jam tracks in our player will be helpful for that, and if you don't want to drop a dime on an actual metronome, search for one online (there's a good one at Metronomeonline.com). The numbers you see on a metronome represent pace or "tempo," which is measured in beats per minute, often abbreviated BPM.