Updating a table after adding Index
===

I am designing a database using SQLExpress.
I have a table which has three columns. The table looks as below.

    CREATE TABLE [dbo].[dummy](
	[id] [int] IDENTITY(1,1) NOT NULL,
	[someLongString] [text] NOT NULL,
	[nameHash] [binary](20) NOT NULL,
     CONSTRAINT [PK_dummy] PRIMARY KEY CLUSTERED 
    (
    	[id] ASC
    )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
    ) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]
    
    GO

I already have some data in this table. Whenever I want to add a new row, I first compute a hash on `someLongString` and query the table to see if a row with this hash already exists. As the table size grows, this query talks longer time and hence I plan to index it by the `nameHash` column.   

Can some please suggest how to do this in SQL Server Management Studio. Also, after adding this index, how do I index the existing rows in this table ?