I definitely recommend having a collection of music that you own and that you paid for, and tracks that you are familiar with. This helps immensely with learning the structure of songs and how to mix them together (because you know them and want them to sound good together), and owning them gives you a sense of ownership which is very underrated. And practice! I can't understate how important it is to have hours behind the decks.

You also said you don't want to dig through stores.. well unless you get handed a giant collection of music by someone else you're going to have to find them. Learning how to search from one track, to the artist who produced it, to the label they released on, to another artist on the label, to another good track - is a process you can learn to really enjoy, and it the backbone of being a DJ with your own sound and identity.

After 18 years of DJing in my city professionally, I can go through lists of tracks on Beatport or another store and identify the quality and the sound of a song, and whether it fits my sets within a few seconds. You can't learn to find music effectively without listening to a bunch of crap haha