how do i create a second email account?

To create a second mail actual account you will need to create a new iCloud account, by creating a new Apple ID at http://appleid.apple.com and then signing into System Preferences>iCloud with it and enabling Mail in the checklist, at which point you will be asked to create a new @icloud.com address.

However if all you want is a new email address you can add up to three to your existing iCloud in the form of 'email aliases' - these are additional addresses (not accounts) which deliver into the same inbox as the main account. (In fact it's a good idea to give out alias addresses, rather than the main address, because if they attract spam you can easily change them.)

You should be aware before you start that once you've created an alias you cannot turn that address into a full iCloud account or move it to another account.

More information on aliases here: http://help.apple.com/icloud/#mm6b1a490a

Once you have created the alias you can choose what address to use as the From address when you create an email.

On a Mac, in Apple Mail Preferences>Accounts, the aliases should appear in the 'Email address' field - if it doesn't, add it after a comma. Then when you create a message there is a drop-down menu under 'From' which will let you choose what address to use.

In the icloud.com website Mail page, when you create a message you can select your From address in the same way.