Create a new table.
Add a field, make it integer, and set Identity to true.
For example:
CREATE TABLE [dbo].[MyTable](
[Id] [int] IDENTITY(1,1) NOT NULL,
[UseName] [nvarchar](50) NOT NULL,
[JoinDate] [datetime] NOT NULL,
[Visits] [int] NOT NULL,
CONSTRAINT [PK_MyTable] 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]