how to hide main form before login?

[nebrom]

i amd developing windows application using C#. i want to add login form to my project so that i can only see the main form after i successfully loged in.
so how can i do that. the name of my main form is: hrmainfrm
my login code is:

 private void vblogin_Click(object sender, EventArgs e)
        {
           
            dbcon.Close();
            dbcon.Open();
        string qry;
        SqlDataReader dr;
        qry = "select username,password from userstbl where username='" + txtuserid.Text + "' and password='" + txtpwd.Text + "'";
        SqlCommand cmd = new SqlCommand(qry, dbcon);
        dr = cmd.ExecuteReader();
        if (dr.Read())
        {
           //what  to do here if login is success
            
        }
        else
        {
         lblerr.Text = "Enter valid UserName/Password";
        }
        dbcon.Close();
    }

i am beginner please help me