Before purchasing bitcoin, you'll need to set up a wallet as a place to store them.
Wallets can be on your mobile, desktop or even a physical hardware that you can carry with you. When you set up your wallet, you'll have what is know as a private key which functions as the password to access your bitcoin. As you complete a transaction, you'll then receive what is known as a public key, which connects you from one bitcoin user to another.
It's really important that you have access to your bitcoin wallet, as bitcoin transactions are irreversible. If someone gives you a wallet address to send bitcoin to, it's possible that you will not see these bitcoins again. Never purchase a bitcoin wallet on eBay - someone may be attempting to sell you a wallet that has been tampered with to steal you r bitcoin.
Keeping your bitcoin secure is really important, so make sure to keep your private key somewhere safe.