how to add column for existing table

Try like this

CREATE TABLE dbo.Employees
(
  EmployeeID int IDENTITY (1,1) NOT NULL PRIMARY KEY NONCLUSTERED
)
GO

ALTER TABLE dbo.Employees
ADD
    FirstName varchar(50) NULL
    ,LastName varchar(50) NULL

If you have already added data into the table, you need to ensure that the column you add takes this into account if the column you want to add is none nullable. As others have said, you need to use the ALTER TABLE command, but as I've said if it can't be NULL, you need to apply a default when you create it. A practical example would look like this:

ALTER TABLE MyTable
ADD MyColumn DATETIME DEFAULT GETDATE() WITH VALUES NOT NULL

Use ALTER TABLE[^] command
or "Modify" from popup menu.