To be a DJ, you need to stay up to date with music trends and their are many ways to do this. A great idea is to become familiar with what other DJs are doing and find some inspirations. I suggest finding sets DJs have played (live / radio / even podcasts), listen to them (see what your inspirations are playing), listen to how they transition between songs and even get tracks listings (this is a great way to find new music and become familiar with DJs / what’s popular).
By going to the above sites you can find the sets your favourite DJs are playing, listen to the set, see the tracklistings and by doing so, you become familiar with new music, new artists and basically have the ability to see what DJs at the top of their field are doing.
You can also listen to the radio, spottily, follow artists on Soundcloud, go to lots of live gigs and whenever you hear a song you like, hit SHAZAM on your phone to get the track name and keep notes of the tracks you like and you feel people would resound well to. Then, next time you are downloading music, find these tracks and add them to your library.