How to check the user name already exits in the table by using stored procedure

create proc sp_user
(
@userid int,
@username varchar(20)
)
as 
begin
insert into tbl_ins(userid, username) values(@userid,@username)
end

This is your code, i have some suggesstion to you
1) Never create a procedure with The starting letter of SP_ see the reason here[^]
2) where is the try catch?
To answer your problem you have two way
1) Create a unique constraint(Key) on the column USername in the table tbl_ins
You don't need a extra crap code here?
2)(well some one say that i will maintain my code in my Proc only)inside the proc with extra code here

CREATE PROC sp_user (@userid INT ,@username VARCHAR (20))
AS

BEGIN
    IF EXISTS ( SELECT 1
                FROM   tbl_ins
                WHERE  UserName = @username )
    BEGIN
        RAISERROR ( 'UserName %s Already Exists.', -- Message text.
          16, -- Severity.
          1, -- State.
          @username
           );
    END
    ELSE
    BEGIN
        INSERT INTO tbl_ins
          (userid, username)
        VALUES
          (@userid, @username)
    END
END

i think now you are clear, reply me what's your solution either 1 or 2