First, whistle Disney's "It's a Small World After All." Can't whistle? Hum it.
It's a small world after all(Don't recognize it? Do "Amazing Graze" or "Yankee Doodle" instead.)
It's a small world after all
It's a small world after all
It's a small, small world
Next, whistle the beginning of Bartok's Second Piano Concerto, which Wikipedia describes as "one of the composers more accessible compositions for audiences."
Bet you can do the first but not the second. You can't whistle a tune you don't know. To play a tune, you need it in your head.
Go out and buy a bunch of recordings of dance tunes, then play them until you can whistle one of them. (If you don't live by yourself, get earphones.) That's the next tune you want to sit down to play.
None of this, "Buy a book of fiddle tunes and try sight-reading them." That'll teach you to read, not to play.
After you try playing it, spend a few days with it stuck in your head. Whistle it. Hum it. Apologize to your girlfriend when she says, "Could you whistle a different tune?" If you're going to play melodies that the folks listening to you can't get out of their heads, you need to pick the ones you can't get out of your head, either.
When you go back to play it tomorrow and the next day, you'll discover it lays under your fingers more and more naturally each time. You're not thinking about where the tune goes next, you're thinking about how to make your hands play the song in your mind.
When the fiddler says, "Let's play 'Red-Haired Boy,' " you won't say, "How's that go?" because it will spring from your head, fully formed, like Athena from the head of Zeus.
When someone in the jam turns to you to start next tune, you'll say, "Sure. I have one."
You've caught an earworm.
I bet you're still humming "It's a Small World."
Play the chorus twice and it's an AABB, thirty-two bar tune.
It's a world of laughter A world of tears It's a world of hopes And a world of fears There's so much that we share That it's time we're aware It's a small world after all
There is just one moon And one golden sun And a smile means Friendship to ev'ryone Though the mountains divide And the oceans are wide It's a small world after all
It's a small world after all It's a small world after all It's a small world after all It's a small, small world
It's a small world after all It's a small world after all It's a small world after all It's a small, small world
"It's a Small World" by Richard M. Sherman and Robert B. Sherman