Unable to set address?

Hi I have had `geth` installed on my mac and got this from running `eth.coinbase`
```
Error: etherbase address must be explicitly specified
    at web3.js:3104:20
    at web3.js:6191:15
    at get (web3.js:6091:38)
    at <unknown>
```

What do I do? Is there any way to set the address?