You can set any values you want for rpcuser and rpcpassword in bitcoin.conf. Those values will be your username and password when you will be connecting to your bitcoind through HTTP JSON RPC. Also be sure to set other important values in .conf file, like server, rpcallowip and a few others to ensure your server is running correctly and securely.
- Testnet on the bitcoin wiki.
- Learning How to Use Bitcoin: A Beginner’s Guide to Using the Bitcoin Testnet