Since you didn't specified the version of Apache I will assume apache 2.4 in a Redhat/CentOS O.S.
I will assume that you know how to create a certificate or that someone has given the certificates and the keys files that you gonna need.
First you need to install and enable mod_ssl.
Install:
yum install mod_ssl
Edit /etc/httpd/conf.modules.d/00-ssl.conf and uncomment the following line:
LoadModule ssl_module modules/mod_ssl.so
Second edit /etc/httpd/conf.d/ssl.conf and ensure at least the following lines:
Listen 443 https
SSLPassPhraseDialog exec:/usr/libexec/httpd-ssl-pass-dialog
SSLSessionCache shmcb:run/sslcache(512000)
SSLSessionCacheTimeout 300
SSLRandomSeed startup file:/dev/urandom 256
SSLRandomSeed connect builtin
Mutex sysvsem default
SSLCryptoDevice builtin
For every VirtualHost that you configure you need to include this:
<VirtualHost *:443>
ServerName your.domain.com
SSLEngine on
SSLCertificateFile /etc/pki/tls/certs/your.domain.com.crt
SSLCertificateKeyFile /etc/pki/tls/private/your.domain.com.ersa
SSLCertificateChainFile /etc/pki/tls/certs/yourCA.crt
...
# Lots of configs
...
</VirtualHost>
And test your configuration with:
apachectl configtest
Then if everything went ok restart apache.