The screen shot below shows you how to choose which account you want your message sent from:
Once you have set up the additional email address(es), you can decide which one you want as your default by clicking Settings (gear icon), then the Accounts Tab:
You can also choose to Google reply using the email address to which the email was sent on this tab.
If you need to change your sending address while sending (for example, on a brand-new message), once you have more than one email address set up, you will see a drop-down arrow next to your default email when you click in the addressing portion of the message:
Click the arrow to choose the correct email address: