Everyone talks about keys, what exactly is a key. Where can I purchase one and how does it work.
My answer is that you don't need to buy a key, it's randomly generated when you create a wallet to use. I recommend you need to learn how to use the wallet as well as store the wallet code, the seed code. That's the key to help you get into the current wallet, I think you still don't know how to own it. If you don't know how to use it and don't have the keys, chances are your assets will be locked and you won't be able to use them, move them or sell them.
It's better to purchase a hardware wallet than using the free Bitcoin wallet if Op can afford it. There's a feature that has an advantage upon using a hardware wallet because HW can protect itself against malware infection from the computer. Here's a thread of a
[BIG LIST] Hardware wallets (80+) and I highly recommend Trezor or either Ledger Nano wallet.
As suggested by most users above, Electrum is the best since it's free to use but there's a crucial thing that you must understand first. Watching this
Youtube video might help you in advance.