The number one most important skill is to pick the right music in the right moment. Period. Online many people will tell you about beatmatching, transitions, the best gear and all that, and sure, it is good if you can do that or buy that. But better a DJ who picks good music with crude transitions than one who picks boring music with stellar transitions. My digital sets I do with my laptop, Mixxx and an audio interface, no controller, just keyboard shortcuts.

Even if you just crudly made hard cut-transitions and don't match beats you can still give people a great evening if your music selection and the order in which you present it is good. I once had people partying till early morning with my phone (android/poweramp) as a source, as it wasn't a planned gig.

That means you (should) invest significant time in listening and sorting your music and have an clear idea which types of DJ set you like. A lot of this will depend on the style(s) of music as well. Putting on dub is very different from straight techno which is very different from let's say playing an 80s post punk set or something entirely eclectic. And a lot of that depends on the venue, the crowd etc. A good DJ will react to the circumstances around them.

As for Mixxx, most of the metaphors and parts of the UI in DJ software is derived from vinyl DJing. A crate would be a literal crate filled with vinyls. Since vinyl is heavy as shit, bringing all your music was/is not an option and DJs packed literal crates, for a specific gig, etc. The two Decks on Mixxx are refering to the record players (typically two "Decks") you would have with vinyl. In the middle you have a mixer section, just like the mixer between the two record players.

Your task is (a bit simplified): selecting a song, playing it, selecting another song and then transitioning to that song ad infinitum. So you load up a song in the left deck hit play and have it play and you move the crossfader to the left so only the left deck can be heard. You then search for the next track while the song is playing and load it up on the right deck. Mixers typically have a PFL button (pre fader listen) that allow you to "privately" listen to channels even if their volume fader (or the crossfader) is turned down (this is why DJs have headphones). You basically listen with one ear to the the thing the audience hears and with the other to the next song.

Now the song on the left deck is nearing its end and you have another song on the right deck, you try to make the two align in time, tempo, frequency, pitch in such a way that the transition sounds good and then you use the crossfader to blend over. Or you could just wait for song A to finish and then hit play on song B if you don't care too much about the transition. Or you could mix only the bass from song A with the hihats from song B, add effects, shout something into the mic and then play a song from a third deck, while all while listening to song requests.

For the tempo matching Mixxx has a button that does that for you on each deck, not like if you DJ with vinyl where you have to do that by ear. The mixer has an EQ (changing the level of Bass/Mids/Highs at fixed frequencies) and a Filter (cutting away high and low frequencies with variable frequency, allowing you to avoid situations where two basslines/hihats are playing at once during a transition.

You can go arbitrarily complex, but as mentioned at the beginning, music selection and situational awareness is king, everything else is just details as long as you don't make jarring mistakes with jumps in level or prolonged gaps of silence etc.