I create few sites collection and one search center when I search for people result shows nothing.I have no clue where i am doing wrong.
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You probably don't have User Profile Service configured properly for this.
Here's how to check: SharePoint 2013 People Search Result Empty
If you need full instructions, you can read this tutorial:
Let me know if you need help or have more questions.
There are tons of things can go wrong but moving forward. please validate these prerequisites details MSDN article.
A Managed Metadata service application must be running in the farm.
User profile synchronization must be configured in the farm. If this has not been done yet, at a minimum you must complete the following procedures that are described in Synchronize user and group profiles in SharePoint Server 2013:
Setup People Search
After validating all the prerequisites start Setting up people search from this MSDN article.