How to fill the column in attribute table with same values in QGIS?

Use the Field Calculator. If your column is empty and it has to be filled with the string "mystring" you need to select all the rows of your attribute table and than use the replace function:

replace( 'NULL','NULL','mystring' )

If all values are the same, just open your attribute table. choose the column you wish to modify, add the value you want and click on update.

You can also look at a useful plugin called MMQGIS. Where you can export import CSV files....

Instead go to field calculator, click on Update existing field, and then select the column where you have to add the value and in below Expression box Add the value in single inverted comma Ex 'No'