Can I just make up a value for rpcuser/rpcpassword in bitcoin.conf? Or do I have to set one up somewhere?
You can set any values you want for
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
rpcallowip and a few others to ensure your server is running correctly and securely.