Honestly don’t worry too much and just have fun. While jumping in to the online modes is exciting, playing the single player campaign can help teach you how to play so I’d try to play it along side playing online. The single player does have some lore and stuff too.
Also be sure to experiment with the different modes to find out what you like. I personally love Salmon Run and Turf War but Anarchy battles are a bit too competitive for my tastes. It’s worth giving all modes a shot though.
I personally haven’t really noticed many toxic players, but like with any game I’m sure there are some but for the most part it’s just a fun time.
Finally one last tip I’d say is don’t be afraid to take breaks from the game from time to time if you just aren’t feeling it. I play pretty regularly but even I’ve taken a week or so break when needed. You feel so much better when you return trust me.