NES was my first piece of Nintendo merchandise. If you are talking about non-gaming specific, then it would be a t-shirt, likely mario or zelda IP related when I was 10 or so.

I think I had a Nintendo Trapper Keeper (or something similar) in Elementary school. That's the first non-game Nintendo item I can recall.

Does this count?

I think Amiibos may actually be the first bit of merc I've bought looking back.

Ufff, my first piece of Nintendo merch is a Pikachu piggy bank. It made Pikachu noises every time you put a coin in it.

Got this in a kids meal 1989

https://www.videogameauctions.com/wp-content/uploads/2013/10/Mario-Nintendo-1990-vintage-Avon-velcro-wallet-Super-Mario-Bros.jpg

This animated Super Mario Bros 2 wallet was my first. Must have been the early 90s.

I had that same wallet!  That might actually be the first item I had now that I have seen it.

Probably some Pokemon pogs I won playing Yu-Gi-Oh at school.

A Pokemon toy? I remember having a mario t shirt as a kid even though i had never played it