A valid Monero mnemonic seed is a phrase with 13, 14, or 25 random words (multiple languages are supported). Your mnemonic seed has all the information needed to restore your wallet.
If you run into any trouble, running `./monero-wallet-cli --help` will show you the options available to you at wallet startup. Once you're inside your wallet, running the `help` command will list the help for the commands available to you within the wallet.
If your wallet is in `Advanced mode`, the `Daemon settings` page will be displayed. Here you can choose whether you want to start a local node or connect to a remote node. Choose your desired option and click on `Next` button.
If you set a restore height higher than the block height of your first incoming transaction, your wallet software will not detect all transactions of your wallet. In consequence, your balance will be wrong and your wallet software will not work properly.
If you set a restore height lower than the block height of your first incoming transaction, your wallet software will scan unnecessary blocks. In consequence, your initial wallet synchronization will take longer than necessary, but your wallet software will still work.
Monero CLI will then ask you to give your wallet a password and confirm it. Since your wallet will hold your private keys and seed, it is really important to enter a strong password.
Most Monero wallets ask the user to write down a mnemonic seed on a paper backup when a wallet is being created. So it's up to you to remember where and how you stored your mnemonic seed.