There are a lot of things you should know. There are different main weapon classes which are Rollers (which includes Brushes), Shooters, Splatlings, Chargers, and Blasters. Picking a weapon is easy, just find one that you enjoy using and you're good with.
There are a few types of games that you can play. Turf War is the standard mode, where your objective is to have more of the map covered with your color ink. Turf War is the only non ranked mode of gameplay. Ranked mode is unlocked at level 1o. The ranked modes are Splat Zones, Tower Control, and Rainmaker.
Splat Zones is kind of like turf war but it's only a zone (or two depending on map) where you want to ink. Tower Control is where you are trying to get the tower as far as you can by riding on it. Rainmaker is different, it's a weapon that charges and you want to get it to your opponents goal.