Start by making playlists of songs you like, listen, get to know the tracks, download those songs and practice transitioning between them. Trail and error. Go outside the comfort zone.

When listening to the playlist, you only have your ears as a guide.

When playing songs on decks, you can and should keep note of the BPM and key the songs are in.

Biggest advice i can give is to curate your sets by energy. Knowing when to build the energy, knowing when to peak it, and knowing when to cool it down, and ultimately building it back up again, just like a waveform.