If you can hum a tune, any tune, you already have the basic skill that you need. It's called 'audiation' and it is the ability to 'hear' a complete tune in your head without going through your ears. Try it with something like Pop Goes the Weasel of some other song you learned as a kid.

The next step is to make the same thing you can do with your voice happen through your fingers on your instrument. The process is called finger singing and it's a slow process. Even 5 minutes per day of audiating some simple tune and attempting to play it will increase your skills over a period of months and years.

The process is one of decision making. You are comparing the note your fingers played to that note's blue print in you mind's ear. Obviously, there are only three possibilities - too high, too low, just right. By going through through the decision making process over and over again, it gets faster and more sure.

Embrace your first 750,000 mistakes. They're not really errors, they're learning opportunities. The first million errors will make you an expert.