If you have internet in your home, you may already have a Wifi router. Wifi is wireless internet. Ask your internet provider if you have a Wifi router. If you do, ask for the network name and password (or it may be on a label under the Wifi router or on paperwork that came with your internet installation).
Any other Wifi networks the Kindle shows you belong to your neighbors. You cannot use those because it's illegal to steal someone else's Wifi and their network is probably password protected. If you don't have Wifi in your house but do have wired internet, you can buy and add a Wifi router to your wired internet or ask you internet provider (may be an additional monthly rental fee if you ask your internet provider for it).
If you have no internet in your home, then you need to go someplace that has Wifi you can use like Starbucks (for customers), or a friend's house or maybe a public library.
A Kindle requires Wifi to be registered. That is how it connects to the internet and connects to your Amazon account.
If it is not your wifi you can't get it. If it is yours, it should be whatever you set it up to be. The Kindle can pick up WiFi signals from neighbors, but if they are password protected you can't use them without hacking into them.
If you have a modem for wifi at home, the password is usually on a label on the back. That is what the Kindle needs to link to your home wifi.
You get the password from whoever knows it… Are you in a public restaurant or hotel? They can give you the password
From your router! Or from MacDonald's if that is where you are going to connect.