You have an Android device running Android 5.0 "Marshmallow" or better, and it's running low on storage space.
Some modern Android devices don't have a slot for a MicroSD flash memory card. But yours does; it's hidden underneath the (removable) back cover.
You want to buy a MicroSD card; how can you decide which one to buy?
This answer assumes that your phone is running Android 5.0 "Lollipop" or higher, and that your phone lets you move apps to a MicroSD card. (Most ancient Android 4.x phones, and even some modern Android phones, lack support for moving apps to MicroSD cards.)
It's quite possible that you'll eventually decide to store apps on your MicroSD card. Therefore, I recommend that you choose a MicroSD card which is good enough for fast app performance.
Lots of product-review websites offer recommendations. The Wirecutter's recommendation article is a fine choice; it looks like they've been periodically updating it for the past few years.
I'm reluctant to offer a specific recommendation here, because I may not return here to update this answer every few years.
Before storing apps on a MicroSD card: Consider buying a card large enough so that you'll never need to upgrade your current phone to a larger card. This is because, once you've installed apps on a MicroSD card, it may be difficult or impossible to upgrade your phone to use a bigger card.
MicroSD cards store your data on a tiny flash-memory chip. Unfortunately, the profit margins on MicroSD cards can be razor-thin. To cut costs, manufacturers use low-grade flash-memory chips which may be far less reliable than the flash memory built into your phone. See, for example, this source.
MicroSD cards can fail irreparably, without prior warning.
If it only holds photos, music, and videos, a MicroSD card will probably last for years. If you store apps on the card, though, this puts more stress on the card, and it may fail significantly sooner.
If you plan to store any irreplaceable data on your MicroSD card, make regular backups. If the backups are encrypted, keep extra copies of the encryption key in two different places.
You can buy special "high-endurance" MicroSD cards. These may be more reliable, but are often slower. (Source.) I therefore assume it's probably unwise to store frequently-used apps on "high-endurance" cards.
Counterfeit cards can fail, permanently, within an hour or two of purchase. The Wirecutter offers some good advice on avoiding counterfeit cards. One good option is to shop in-person at a trustworthy local store.
There are lots of tools which can tell you whether or not a card is counterfeit. Some such tools are very slow. The fastest tool is ChipGenius (for Windows). If its reported card capacity matches the advertised card capacity, then the card is genuine. Another reasonably-fast tool is FakeFlashTest (for Windows). These tools are freeware. (Source.)
If you bought a counterfeit card, you may or may not be able to get your money back. But it's worth trying.
If that fails, you can try any of the following:
Speed matters. If you wait too long, it may be too late to try to get your money back.