You have a new MicroSD card, but before you use it in Steam Deck – you should format it. How to do it – read in this guide.
Before we start, here is a couple of things I wrote about, and you may be interested if you are new to Steam Deck
How to select MicroSD card for Steam Deck / Best MicroSD for Steam Deck?
Steam Deck managing storage / how to eject MicroSD…
Ok, let’s get on topic.
Insert MicroSD card into your Steam Deck

Power up your Steam Deck if it is shut down.
In Gaming Mode – press Steam button

On menu select Settings…

BE CAREFUL BEFORE NEXT STEP – MAKE SURE THAT THE MICROSD card is EMPTY AND THAT NOTHING VALUABLE IS ON IT – FOLLOWING STEP WILL ERASE EVERYTHING FROM YOUR MICROSD!!
From the left sidebar select System menu – under System Settings, under Format SD card – select Format.
It should be quickly done.

On the sidebar, you can now go down and select Storage – under Storage you should now see your MicroSD card besides your internal drive if everything went well.

And that is that, you now have a MicroSD on your disposal. It will be in ext4 – Linux format, since that is what Steam Deck uses. It won’t be natively readable on Windows PC help is In this guide where I wrote about attaching disks from Steam Deck to PC.