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:
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.