If you already have an iCloud account, and thus an @me.com address, you can add up to three 'email aliases', which are additional addresses (not accounts) which deliver into the same inbox as the main address, and thus work across all devices which are signed into that account. Please see:
Note that an alias address, once created, cannot be moved to another iCloud account, changed into a full iCloud account, nor reinstated if deleted.
If you want an additional address to be accessible on only one device (while retaining access to the existing address) you will need to set up a new iCloud account by creating a new Apple ID. Then you sign in with it in System Preferences or Settings>Mail, Contacts & Calendars so that it will appear alongside the existing account.