Whenever I start a new email in Mail, my .mac account is the default account the application uses to send the email. However, I do not use this account, I only have it set up for the few emails that might come through. My main account is my Google account. How can I set up mail so that when I click the "new email" icon, my Google email address is the default one??
In the Composing pane of Mail's preferences window, look for the pop-up menu labeled "Send new messages from." Select the desired account there.
Note, though, that this won't change the behavior when replying to a message... in that case, the reply will be sent from the account that received the message, unless you specify differently at that time.