How to Find Performance Issue?

Hi,

     I have developed a report program in development server its working fine without any issue but in client side if they are using the program more than 50 user the program getting slow, how to find where the problem is and here my role is performance issue analyst.

     what tool i have to use to find the issue,please tell me the effective way.

Thanks & Regards.

Lokesh Srinivasan.