The Switch has a Plastic Screen that is not very durable and easily can get scratched if you aren`t careful. The best way to protect it from any scratches are those tempered glass protectors. They re easy to apply and are cheap, so go get them rather sooner or later if you don`t want a messy switch.
We recommend this one: https://amzn.to/2meTMxY
One other very important thing to get is a carrying case. The switch is meant to be carried around in backpacks and other places wherever you go. So the first thing you should get is a proper case. Without a switch case you are basically limited to home use, which is, let`s be honest here, besides the point of a portable gaming system like the Nintendo Switch.
There are tons of different cases and which one you get heavily depends on what type of gamer you are.
We can recommend the orzly carrying case if you want something solid for not a lot of cash. Alternatively, you could get a super protective case like the Mumba Switch case. The Mumba case especially makes sense if you have Kids around.
The next important thing to get is flash memory. I recommend the SanDisk Ultra 256GB card at least. Get as much as you can afford to have a stress free experience
The Nintendo only comes with two Joy Cons and not the Pro Controller. If you are serious about using the switch as a Home console you need the pro Controller from Nintendo. This offers a superior experience. Especially FPS shooters are best enjoyed in that way.
Last but not least you should get some decent controller grips. they make holding the switch a much more pleasant thing to do and they even give you a little boost when it comes to controlling games in portable mode.