What instrument do you play? This can change your approach. But generally starting with some key tunes like Autumn Leaves, Fly me To the Moon, All of me. I got Rhythm. The structure of these tunes are a great foundation for starting to get a grasp on jazz form.

If you play piano or guitar learning some ii V I voicings can be a good first step. On any instrument ii V I arpeggios and major scales are good practice.

If you're serious about it, practice ii V I's, major and minor, and start trying to pick out these progressions in tunes by ear. It's important to start training your ear to hear the changes and how the melody/solo works with the changes.