I'm having trouble setting up my email for my Mac, it's version 10.7.5
First, remove his account from the list of accounts in preferences.


Go to Mail > Preferences [command ⌘+,
Go to the Accounts tab [Looks like an @ symbol]
Select his old account
Click the Minus button at the bottom of the list to remove his account [see picture below]





Now, add your email account.  Note: Not to be confused with your Apple ID and password.  Even if your Apple ID is the same as your email, the passwords may be different.  One accesses Apple services (iCloud, iTunes etc.) the other is your email account.