Digital Certificate of Digital Signature?

We need to implement digital signatures on our Offce and SharePoint documents.  I have searched everywhere for details to specific questions I have, but cant find any answers.

We need these signatures to be recognized outside of the company, so I understand I will need to go with a 3rd party....but 3rd party of what?

Is this all correct below? Any clarification would be appreciated

1) I need to purchase a Digital Certificate, correct?
2)What do I do with that Digital Certificate? Do I load it into my local Certificate authority?
3)How do I point all Office applications on everyones PC to look to the local Certificat Authority?
4)Does the Cert somehow check with Active Directory to confirm I am "John Smith" before allowing me to sign?

I spoke to VeriSign, but all they know is to give me the certificate. They are of no help on how to implement that.

Is there any documentation on what to do with the certificate once I have it? I think that is the most confusing part.

Thanks