User!UserID displays the name of the user (domain name or other credentials passed in) executing the report, assign this field to a parameter in your report.

report parameter: @userid

value: =User!UserID

With some string manipulation, use this parameter in your main query returning results

SELECT

empNo,

empName,

esal

FROM table

WHERE empName = @userid

If empName != @userid you will have to cross reference a user table (if you have one) with the userID variable. You'll need another dataset and another report parameter.

SELECT

empno

FROM userTable

WHERE networkID = @userid

report parameter: @empNo

value: =First(Fields!empno.Value, "dataset name")

Then select your main dataset

SELECT

empNo,

empName,

esal

FROM table

WHERE empNo = @empNo

This should get you something to start with, it will require some tweaking on your part depending on your database.