If you prefer to download games, then as the above poster says buy a Micro SD Card as soon as possible. You'll probably get away with most Class 10's, but I'd always recommend SanDisk as they're probably the biggest brand and they're not too pricey now.
256gb:
https://www.amazon.co.uk/SanDisk-mi...keywords=Micro+SD+Card&qid=1602145224&sr=8-10
128gb:
https://www.amazon.co.uk/SanDisk-mi...&keywords=Micro+SD+Card&qid=1602145349&sr=8-3
They're fairly priced, even if you plan to buy a lot of physical games I would still suggest getting a 128gb and putting that in anyway at £16. I'm sure you will find some games you want to download as well.
If you plan to play on the TV a lot, I would also suggest getting a Pro Controller. The Joy-Cons + Grip is a reasonable substitute but I would not suggest it for long play sessions and as a permanent method of play. Sadly, the official one is quite expensive but I think it's one of better controllers out there personally. Zavvi has it for 'pre-order' at the moment so I assume they are just awaiting more stock. £50 on there which is the best I've seen.
Pro Controller:
https://www.zavvi.com/accessories-n...VzLTtCh1l_AQsEAQYASABEgJaFvD_BwE&gclsrc=aw.ds
If you plan to play online, you'll need an online subscription as well. Don't purchase it directly on the eShop, buy a code from ShopTo. You can do a single account for a year for £15 or a Family Pass (8 Accounts) for £25 a year.
Nintendo Switch Online:
https://www.shopto.net/en/switch-membership/
Those are some things besides games I would suggest at least considering, although if you plan to get a lot out of your Switch I'd put them closer to the 'necessary' column.
As for games, what kind of games do you usually play? Is it Nintendo exclusives you're mainly buying this for?
You can't go wrong with what
@Dirty Schwein listed, some others I'd say are:
Pokémon Sword/Shield - If you liked the Pokémon games, it's more of the same for the most-part.
Donkey Kong Tropical Freeze - A great alternative to Mario / Yoshi / Kirby as a side-scroller.
Fire Emblem 3 Houses - Can be difficult but if you enjoy tactical games it's up there with the best.
Super Mario 3D All-Stars - If you can find a physical copy get it, if not still download it.
Astral Chain - If you like the Devil May Cry games and a bit of investigation / police antics this is amazing.
Bayonetta 1/2 - Two games in one, both great but particularly the second, see above similar to DMC.
Animal Crossing - Watch some videos of this as it's definitely not for everyone, but everyone seems to be buying it
I would be slightly wary though as there are big rumours again that Nintendo may release a new model around March-July time... Not sure if that's something that would bother you if you purchase one this year.