Watch the Day 7 online guitar lesson by Ravi from Learn Guitar in 21 Days
The "one-four-five" progression is the single most common sequence of chords in popular music. The theory behind it is beyond the scope of this course, so for now, just take my word for it!
Now that you have a "I-IV-V" in the key of A (meaning chords A-D-E), you can play hundreds upon hundreds of songs. Virtually all blues songs are based on this chord progression (hence, this progression is often simply called a "blues") as are many folk, rock, and pop tunes.
Check out my examples, and then look at the song list and see how your repertoire just increased exponentially!