Let's learn to play "Row Row Row Your Boat" in the key of G major. We aren't going to use sheet music, we are going to learn to play it by ear. When you are trying to learn a song by ear it helps to sing it first. You don't have to sing it out loud, you can sing it in your head. Let's try it.
At first, it is the same note for a while; row, row, row. After that though, it goes up for the next two notes. So let's start out on G, play the "row, row, row" part, then move up a couple times for "your boat" and see how that sounds. So it's, G G G A B. That sounds like it does when we sing it, so we know we are on the right track.
Now let's move on to the next part, "gently down the stream". Try to hear for yourself what is going on with the notes in this part of the song. If you can't figure it out, I'll tell you. It's B A B C D. The song then jumps to the G octave for "merrily", which is made up of three notes. The rest of the merrilys follow the G chord down, D, B, G. The song ends off by jumping up to the D and walking back down to the first G for "life is but a dream".
So it's all within that scale. Based on the scale and the chords in the scale, you can add some chords to your left hand for a more full sound. Let's start with the root chord, G. When we get to the second verse, we are playing a note that is found in the G chord, so we'll stay on the G. We'll keep it again for the merrilys because those start off on a G as well. At the last verse, the melody jumps up to a D. The D is in the G chord, but we want to change things up a little for variety. D is also in the B minor chord and the D chord. Let's try both of those. The B minor chord sounds okay, but the D chord sounds more like the end of a song. So let's use that.
For the next song lesson, you can practce The Scientist by Coldplay .