I am not able to find out the current logged users, so that I can able to display on site that “Active Users -> “ , no. of users online.
That's going to be hard to do without some specialized solution. Nothing in the OM exposes who is currently logged in. To implement this you are going to have to do some log parsing or prehaps have a CEWP fetch a outside resource that can track who is visiting your portal. Not espacially hard solutions but it will take some work to put in place.