C# – ASP.Net insert data from Textbox to a database

Im trying to insert data from a textbox to my database, and it throwing me an exception.

Exception Details: System.Data.SqlClient.SqlException: Incorrect syntax near ' test'.

a few details before I presents my code though :
my database called : Movies
my table data called: Users
and I have columns such as : "FirstName", "LastName" etc…

protected void Register_Click(object sender, EventArgs e)
{
SqlConnection connection = new SqlConnection("Data Source=MICROSOF-58B8A5\SQL_SERVER_R2;Initial Catalog=Movie;Integrated Security=True");

        connection.Open();
        //FirstName***********
        string firstName = FirstNameTextBox.Text;
        string sqlquery = ("INSERT INTO [Users] (FirstName) VALUES (' " +FirstNameTextBox.Text + " ' ");

        SqlCommand command = new SqlCommand(sqlquery , connection);
        command.Parameters.AddWithValue("FirstName", firstName);
        //LastName************
        string lastName = LastNameTextBox.Text;
        sqlquery = ("INSERT INTO [Users] (LastName) VALUES (' " + LastNameTextBox.Text+ " ' ");
        command.Parameters.AddWithValue("LastName", lastName);
        //Username*************
        string username = UsernameTextBox.Text;
        sqlquery = ("INSERT INTO [Users] (Username) VALUES (' " + UsernameTextBox.Text+ " ' ");
        command.Parameters.AddWithValue("UserName", username);
        //Password*************
        string password = PasswordTextBox.Text;
        sqlquery = ("INSERT INTO [Users] (Password) VALUES (' " + PasswordTextBox.Text + " ' ");
        command.Parameters.AddWithValue("Password", password);
        if (PasswordTextBox.Text == ReTypePassword.Text)
        {
            command.ExecuteNonQuery();
        }
        else
        {
            ErrorLabel.Text = "Sorry, You didnt typed your password correctly.  Please type again.";
        }

        connection.Close();
    }