Before u do it, read Request SSL certificate.

Create folder and named to ssl into apache2.

mkdir /etc/apache2/ssl

Copy certificates to ssl folder

cp -r YourDomainName_public.crt /etc/apache2/ssl
cp -r YourDomainName_chain.crt /etc/apache2/ssl
cp -r YourDomainName.key /etc/apache2/ssl

Start SSL module.

sudo a2enmod ssl

Edit default-ssl.conf.

vi /etc/apache2/sites-available/default-ssl.conf

<IfModules mod_ssl.c>
<VirtualHost *:443>  
ServerName   #Modify Domain
#Modify these line
SSLCertificateFile /etc/apache2/ssl/YourDomainName_public.crt   
SSLCertificateKeyFile /etc/ssl/apache2/YourDomainName.key
SSLCertificateChainFile /etc/apache2/ssl/YourDomainName_chain.crt

mirror default-ssl.conf to /etc/apache2/sites-enabled

ln -s /etc/apache2/sites-available/default-ssl.conf /etc/apache2/sites-enabled/001-ssl.conf

Reload apache2 config.

/etc/init.d/apache2 force-reload

Restart apache2.

/etc/init.d/apache2 restart

Done.