Play VGC. Singles are a cancerous shit-fest of switching around for half an hour before actually clicking an attacking move while hazards drain your health down.
VGC is the officially supported format of competitive pokemon and the way these games are actually balanced to be played. Doubles are fast paced and require more skill, since pressing the switch button every turn means you eat shit from two enemies at once. As a nice plus, it's easier to avoid shitty Verlisify sets in VGC compared to dog-shit single player stuff.
Getting into doubles with these guidelines is very easy: https://trainertower.com/vgc-2018-resource-index/
and good players/content creators like Cybertron who go through their thought processes as they play and explain why they do certain things helps you learn predictions and team preview just from watching.
VGC is also the "safer" bet to play comp. Pokemon Showdown is a copyright violation and could theoretically be taken down at any given time. VGC is eternal.