I have a PHP Website which is using Parse -PHP SDK.
May I have steps/understanding, what changes i need to do in the project itself?
I have migrated my db already to mongolab.What I need to do next? Where can I found proper guide .
You would need to setup your instance of parse-server. There is a number of options and providers you can use that make this process easy or you can setup your own server.
Some providers ( there are many more ):
The git docs for parse-server will explain more in detail on the setup.
You can also make use of the migration document that gives step to step details on how to setup form start to finish.