Column Name As Variable

Using a stored Procedure, I want to select a record from a Table if a certain string appears in a specified column, so I use

CREATE PROCEDURE spSearcher
@String varChar(20)
as
Select Column1, Column2, Column3 from Table1
where Charindex(@String1,Column1) > 0

which works fine.
But the following doesn't work:

CREATE PROCEDURE spSearcher
@String varChar(20)
@ColumnName VarChar(20)
as
Select Column1, Column2, Column3 from Table1
where Charindex(@String,@ColumnName) > 0


How can I substitute a variable for Column1, so I can pass the name Column1 or Column2, or Column3, as desired?

Thanks in anticipation,

Peter Caspersonn.