if you ran geth correctly on the first run.. you will get an address you keep forever.. well you got to know how to store it.. here is the link ..
https://github.com/ethereum/go-ethereum/wiki/Managing-your-accountscreating an account
$ geth account new
Your new account is locked with a password. Please give a password. Do not forget this password.
Passphrase:
Repeat Passphrase:
Address: {168bc315a2ee09042d83d7c5811b533620531f67}
Listing accounts
$ geth account list
Account #0: {a94f5374fce5edbc8e2a8697c15331677e6ebf0b}
Account #1: {c385233b188811c9f355d4caec14df86d6248235}
Account #2: {7f444580bfef4b9bc7e14eb7fb2a029336b07c9d}
Import private key
$ geth --datadir /someOtherEthDataDir account import ./key.prv
The new account will be encrypted with a passphrase.
Please enter a passphrase now.
Passphrase:
Repeat Passphrase:
Address: {7f444580bfef4b9bc7e14eb7fb2a029336b07c9d}
Account update
$ geth account update a94f5374fce5edbc8e2a8697c15331677e6ebf0b
Unlocking account a94f5374fce5edbc8e2a8697c15331677e6ebf0b | Attempt 1/3
Passphrase:
0xa94f5374fce5edbc8e2a8697c15331677e6ebf0b
Account 'a94f5374fce5edbc8e2a8697c15331677e6ebf0b' unlocked.
Please give a new password. Do not forget this password.
Passphrase:
Repeat Passphrase:
0xa94f5374fce5edbc8e2a8697c15331677e6ebf0b