How to show first parameter value in drop down list as a default value dynamically in ssrs report?

You'll need to set up a datasets to get this done.

Set the managers available parameters to pull from a new dataset.

In the query for this data set, select the available managers, and filter on the accounts parameter for example:

SELECT managerName
  FROM managers
 WHERE accountNumber IN (@accountNumber)

Now, in another Dataset, do the same thing, but this time use a top 1.

SELECT TOP 1 managerName
  FROM managers
 WHERE accountNumber IN (@accountNumber)

And use this dataset as the default value.