Travel case and like $40 bux for 9 parchments and Overcooked. Two great co-op games. I don't use a screen protector and it's fine. And the pro controller is amazing but hey sometimes money is tight at first. You can hold off on that and a memory card for a while
Tempered glass screen protector, pro controller (Xenoblade Chronicles 2 edition if you are into that game), a travel case, portable charger, Hori playstand
Glass screen protector like the amFilm ones on Amazon. Case/shell if you are going to be playing mostly portable. This is the Mumba shell.
Pro Controller is good as well.
Also the screen protector does not need to be glass at all, they're meant for glass screens, the Switch has a plastic screen so it won't break or shatter like a smart device screen at all. If you can get them cheap it doesn't matter but for the extra price there's no point.
You won’t hear this a lot, but the most important accessory besides a travel case is another charger. If you have to unplug your dock to get to your charger every time you travel it’s a waste of time.
Also, get Breath of the Wild. Don’t question this...get it - it’s one of the greatest games of all time.