There are two kinds of certificates available - self-certified (free - but people will have to either accept the certificate or manually install it) or paid for (you buy it from a Certificate Authority and as long as that CA is a common one then it will just work in most browsers).
First you will need a private key:
cd /etc/ssl openssl genrsa -des3 -out private/your.domain.tld.key 2048
Drop the -des3 if you don't want a password (this will allow auto-startup of apache - but is much much less secure). Note - if you are going to purchase a certificate - check how many bits the provider wants you to use.
Now - you will need a certificate signing request (CSR)
cd /etc/ssl openssl req -new -key private/your.domain.tld.key -out certs/your.domain.tld.csr
This will prompt you for X.500 information.
Country, Company, Organizational Unit etc you can set as you need.
Common Name (CN) must be the domain you wish to protect. This is a very important point (especially with purchased certificates). It should be the fully qualified domain name. For self-cert you can specify *.domain.tld for a domain wide one (this may well work with purchased certificates but most CAs want to charge per site - rather than per domain).
This CSR can now be sent to a CA for signing. Once signed - the certificate will be returned to you - stick it in /etc/ssl/certs/your.domain.tld.crt.
You can of course sign your own certificate. This has the benefit of being free - but the drawback that users will either have to install your certificate manually - or - every time they access the site they will have to approve the use of the certificate.
To sign your own certificate run the following:
openssl req -new -x509 -key /etc/ssl/private/your.domain.tld.key -out /etc/ssl/certs/your.domain.tld.crt -days <n>
where <n> is the number of days the certificate should be valid for.
Now - we need to install it inside apache2.
Will enable the module.
Make sure that /etc/apache2/ports.conf includes Listen for port 443.
Now - either in apache2.conf - or in one of your virtual host files (make sure that the port in the virtual host file is also 443) add
SSLEngine On SSLCertificateFile /etc/ssl/certs/your.domain.tld.crt SSLCertificateKeyFile /etc/ssl/private/your.domain.tld.key
For more info on why only one NameVirtualHost virtual host can have SSL on a server (and possible workarounds) - see http://httpd.apache.org/docs/2.0/ssl/ssl_faq.html#vhosts