Not an expert in ubuntu
Can you tell me the steps i can follow them and do the changes required
The short answer is that this isn't easy to do with Bitcoin-QT right now. You can try a web wallet (such as blockchain.info) which lets you select the particular address you're sending from. I don't know how it handles change, however.
Edit: Here's a link to the Coin Control thread, where they appear to have some binaries available:
https://bitcointalk.org/index.php?topic=144331.0