Note : To setup SSL with WordPress you must have SSL installed from control panel . ( If you missed , Pleas install first ) Learn How to install SSL
Step 1. Login to your WordPress admin panel . Like http://yourdomain.com/wp-admin/
Step 2. go Plugins -> Add New

Step 3. Search SSL . You can see Really Simple SSL on list click on install . and then active .

Really Simple SSL
Step 4. ( Final Step ) , After clicked active on plugin you can see new page will come . just click on Go ahead, Active SSL . It will logout and ask for re-login