If your Blue is not initialized go to https://www.ledgerwallet.com/start and follow instructions.
Once your device is initialized:
To find your Bitcoin address (required to receive a payment),
1/ Connect your Ledger device and power it on
2/ Enter your PIN code
3/ Launch your Bitcoin Chrome application on your computer
4/ Click on “Receive” on your Chrome app, a popup opens
5/ Copy the “Bitcoin address” which is displayed
Please note that your Ledger wallet generates a new address each time you want to receive a payment, thanks to the Hierarchical Deterministic (HD) support. Each address generated by your wallet is yours forever, you can use them several times or just once at your convenience, it won’t cause problems – even if it may be safer to use each address only once.
6/ Paste this address where you are required to
While receiving payments, your Ledger device can be disconnected as your transactions are tracked on the Blockchain. These transactions will be synchronized on your wallet when you open it.
Source: www.ledgerwallet.com
Bitcoin Wiki 