You can go about it a few different ways
So here’s the thing about Super Smash Bros. Ultimate — you only start with a handful of fighters! Yep I know, it sounds rough, but in the grand scheme of things you’ll have plenty of time to experiment with the entire roster after spending a day (or less) getting them.
There’s a few ways you can go about it.
The old fashioned grind
Every single fighter in Smash can be unlocked in its standard mode over time. The quickest way to grab them all is to just create a one stock bout and go at it with a friend: or, less eloquently, against a CPU and just sacrifice yourself.
Note that there’s a timer on when fighters can be unlocked. You can try to circumvent this by resetting the game entirely, or by legitimately playing through a ton of quick versus games. The choice is yours.
Just know that you still have to fight and beat incoming new characters to unlock them. If you lose, you’ll get the chance to challenge them again from a main menu option after a short period of time (roughly 10 minutes).
Through the campaign (my suggestion)
If you’re the primary Smash player in the house and are going to immediately jump into a solo mode, you’ll want to unlock fighters by way of World of Light.
The thing, you’re able to unlock characters double-time by playing the World of Light campaign. Fighters will appear as encased blue emanating trophies on the world map: if you defeat them you’ll unlock them game-wide. Also, World of Light runs on a counter similar to the old-fashioned away above. By backing out of the mode entirely (out of the save file screen too) you’ll periodically trigger another roster unlocking encounter.
It took me approximately 15 hours to finish World of Light with some exploration of ancillary areas, but I had everyone unlocked around 10 hours into that save file. If you back out enough you can do it considerably quicker in a non-mindless fashion.
The Classic way
Classic Mode unlocks specific characters after completion depending on who you queued up Classic with.
Each Classic run should take around 10 minutes, roughly the amount of time the unlock timer is for fighters (if you don’t reset). If you’re not into World of Light you can go this route but it’s considerably slower than anything else.