Follow below steps to configure your Magento store with SSL:
Go to System -> Configuration
Click on Web under the General
Select Secure tab
Now, under the Secure tab you will find two options as;
Use Secure URLs in Frontend -> Yes
Use Secure URLs in Admin -> Yes
Change both options to Yes.
Below reference links will help, if you stuck anywhere;
How to enable https for my magento site
https://www.siteground.com/tutorials/magento/magento_ssl.htm
https://community.magento.com/t5/Installing-Magento-1-x/Valid-SSL-certificate/td-p/20688