In order to send encrypted information to someone, you'll need certain information from them first. Also, if a recipient wants to verify an electronic signature then they have to have special information about the sender - the person signing. This information is held in an individual's digital certificate.
A digital certificate acts much like an electronic passport, verifying your identify and confirming your rights to access particular electronic information and services.
You can obtain a digital signature from any of a number of approved certificate authorities. These organisations are sometimes known as 'trusted third parties'.
Certificates are available for both individuals and businesses - approved versions are often available on a monthly or annual subscription basis and the cost will depend on the number of signatures you require. You need a separate digital certificate for each e-mail account you want to send signed e-mails from.
Digital certificates are available online and, depending on the level of security you want, should be e-mailed to you within a day of submitting the necessary information to prove your identity.