How to preselect only few fields(columns) on page load. Then user can select any columns from fields tool bar menu.

Hi Manoj,
 
Thank you for posting your question.
 
To specify the fields which you want to be displayed on page load, simply define them in the Slice object inside of the pivot table report. 
 
Here's a step-by-step tutorial on how to define a slice with property explanations & detailed examples: https://www.flexmonster.com/doc/slice/
 
Please let us know if this helps.
 
Regards,
Mykhailo