Jesus, Rick, now they know about my travels and my right hand! Stop!!! (A lady once came up to James Joyce and said, "May I kiss the hand that wrote Ulysses?" and Joyce replied, "No, it did a lot of other things too!")
Rick happily didn't point out that I play basically like a mentally challenged horse. And that he is extremely patient as a teacher.
Anyway, as a late dope, I thought I might add one thing to this thread -- a very little music theory can help a ton. One reason I was stuck for so long was I couldn't organize all the hundreds of songs around me, and the screwing around on the piano I was doing. I learned all these chords to all these songs, but they didn't mean anything -- I just played what was there. Then one day, I sat down and learned some elementary theory, and Jesus is it simple!
Rick mentioned G, C, and D7, but what he didn't say was that you find these together in hundreds of songs, because they are in the same key! And they are related!! If you know all this, move on, but if not, hang with me for a second. Here is the relationship. If the song starts with a G chord and ends with a G, it is likely in the key of G. So call the G chord a one chord ( I). The rest of the chords in the key are built on the bottom notes (G,A,B, etc) as you go up the scale. A C chord is the IVth in the sequence, and the D7 is the Vth (actually V7), called the dominant (the little 7th is to push the chord in the direction of the original chord G which is where the song is heading) . The one chord (I) is also called the tonic chord, and most Western music since 1700 goes from the tonic (I) to the dominant (V) and back again. Like Beethoven's symphonies and virtually all songs. So in the Key of G, you will get G, C, D7 (the I, IV, V7). There are different patterns where the IV chord comes in to supplement the V. This shaping of the tune works in all other keys too! If you have a song in the Key of C, C is I, F is IV, and G7 is V7). As the keys get more complicated, there are flats and sharps, but that is the main idea. Lots of folk songs stick in G or C. Once you are started in a folky type of G song, you can usually bet that it will work towards a D7 and out again to G. This doesn't work for complicated tunes or jazz, but lots of tunes. You will also get IIs and sometimes IIIs, and occasionally weird chords from somewhere else: but mostly the chords are predictable Is, IVs and Vs!! Lastly: if you go past the V to the VI (in G that would be E), lo and behold you will usually find it showing up in the song as a minor (Em). You can find out what a minor is on your own. Anyway, this is the relative minor of the key you are in, and tunes often migrate into that particular minor key, just to be different. G tunes migrate into Em, C tunes into Am, D tunes into Bm, and then back out again!!

If I had known that whole last paragraph 20 years ago, I would have saved years and years of messing around on the guitar to no purpose.
Moral: Reinvest in music education in schools!!!!!!
Yours, Peter T.