Setup wallet

Run wallet -h to see all available command line switches.

Setup your seed

Wallet is deterministic and the only thing you need to set it up is the seed password.
As long as you remember the password, you do not need to backup the wallet ever.
You can either enter this password each time when running the wallet, or you can store it in a file called .secret and never be asked to enter it again.

You do not need to use a memorable password. Instead you can create a random secret seed, e.g. using a command like this:
    cat /dev/urandom | tr -cd a-zA-Z0-9 | head -c 27 > .secret
Note: using a password that you cannot memorize, make sure to have a backup of your .secret file.

Export public addresses

After you setup your wallet in a secured environment, you should export its public addresses.

In order to do this, just run wallet -l and your wallet's public addresses will be written to wallet.txt.

Security precautions

Make sure that the disk with .secret file is encrypted.

The seed password is the key to your entire wallet and the function used to calculate it is double SHA256*, which is pretty easy to compute.
That is why it is very important for you to make sure that the password you're choosing will be resistant to brute force and dictionary attacks.

If you choose to use a memorable seed-password (aka brain wallet), make it at least 20 characters long, preferably more than 30. Use both cases of letters, digits and sepcial characters. Try to avoid using words that can be found in a dictionary and never use sentences that can be found in Google.

If you decide to write down the password, because you are affraid of forgetting it, try to describe it as a puzzle that only you can understad. Avoid writing down the password as is.

Are brain wallets less secure from those based on a random number generator?
It is debatable.
There are people claiming that brain wallets aren't secure because brains cannot be a good source of entropy.
If you have such a brain, make sure to consider this risk (e.g. use a random generator to create the content of the .secret file).
Otherwise coming out with a memorable though uncrackable seed, gives you quite a secure and convenient solution, as you get rid of backups that are a week point of the wallet's security. On top of that you never have to carry the wallet file with you.

*) Wallets version 1.10.1 (or higher) support using an optional scrypt function to convert the password into private keys, making your brain wallet far more resistant to bruteforce attacks. See the scrypt config value (inside your wallet.cfg file) for more info.

Importing other private keys

You can import keys from your existing bitcoin wallet, as well as keys generated by other tools (all kind of key/address generators).

The key that you want to import must be in base58 encoded format, which looks somehow like 5KJvsngHeMpm884wtkJNzQGaCErckhHJBGFsvd3VyK5qMZXj3hS. To export a private key from the official bitcoin wallet use dumprivkey RPC command. To import such a key into your Gocoin wallet, just store the base58 encoded value in a text file named .others (each key must be in a separate line). You can also place a key's label in each line, after a space.

The imported keys will extend the key pool of the deterministic ones (that come from your password-seed). After Importing each new key, you should redo wallet -l to get an updated wallet.txt for your client.

Optional: setup your node with the wallet file

Having wallet.txt file generated at the wallet machine, use Wallet page of WebUI and store the content of this file as a wallet in your browser. You can choose any name you want and you can change labels for the addresses.