I'm still very new to Splatoon, have only played a couple of matches (enough to get LV3), so pretty much everything is new territory for me. Should I try and complete the single player aspect of the game and amiibo challenges first before dedicating a lot of time to multiplayer?
When I started the game, I certainly had in mind using story mode for getting used to the basic controls. Though of course you're going to be spending even more time in any case getting used to the basics in turf wars and even when you start out in ranked.
To make a (perhaps) loosely related point in regards to weapons in particular:
I met someone who is ranked highly as a sniper, and indeed plays very well. She told me how the Hero Charger Replica challenges from one of the Splatoon Amiibo made her rethink her initial decision to stay away from sniper rifles, and to spend more time trying to get used to them. She now swears by them. (Relatedly, I haven't tried out the Amiibo challenges yet, but I imagine they would certainly be more helpful in getting used to basic controls before multiplayer matches simply due to the added restrictions and more weapons to try out under those restrictions.)
Similarly, I stayed away from brushes, perhaps partly because of how people said they are terrible weapons, but after seeing players do very well with them and swear by them in the S and S+ rank range, I gave them another shot, and now I love brushes, too.
In other words, we may not initially know what weapons will best suit us, and indeed some of the ones that initially seem the least likely candidates may turn out to be our favorites, after all. And we find this out simply through experiencing these weapons for ourselves, whether by trying out Amiibo challenges, watching Splatoon streamers on Twitch, etc., and then experimenting with them more in turf wars, and then using them in ranked mode when we gain enough confidence.