This site will help you with ear-training:

you have to do interval training and commit to playing music by ear. at first, it’s really hard; seems like you’ll never be able to do it. but after a while, and if you keep up the interval training, you will become more intuitive. for me, what i usually do is figure out what the chord progression is; after that, the melody usually just comes.

It’s called ear-training, or solfege (do-re-mi, etc.)

There are books for beginners, with progressive exercises. You can do some of this on your own, but will eventually need a knowledgable friend to ‘test’ you – very much part of the practice.

Best, Petr B.

You need to learn how to judge the interval between each note. Most likely this will take some time to learn and you’ll have to listen to each song or work more than once in order to play it by ear.

base you ideas from the distance from 1 note to the next–think in terms of interval relationships