Go to Settings > Mail, Contacts, Calendars. Scroll down to the Mail settings and change the Default Account to the one you want.
In Mail, a new email defaults to whatever inbox or account you are currently viewing. It only uses the Default Account when you are viewing the All Inboxes folder. It might use the Default account for the VIP folder too. I'm not using it, so I've never checked.
At anytime, if the new email shows the wrong From account, all you have to do is tap the address to get a list of all your accounts, and choose one.