var sql = "INSERT INTO myTable (myField1, myField2) " +
          "VALUES ('" + someVariable + "', '" + someTextBox.Text + "');";

var cmd = new SqlCommand(sql, myDbConnection);
cmd.ExecuteNonQuery();

var sql = "INSERT INTO myTable (myField1, myField2) " +
          "VALUES (@someValue, @someOtherValue);";

using (var cmd = new SqlCommand(sql, myDbConnection))
{
    cmd.Parameters.AddWithValue("@someValue", someVariable);
    cmd.Parameters.AddWithValue("@someOtherValue", someTextBox.Text);
    cmd.ExecuteNonQuery();
}

var sql = "UPDATE myTable SET myField1 = @newValue WHERE myField2 = @someValue;";

// see above, same as INSERT

var sql = "SELECT myField1, myField2 FROM myTable WHERE myField3 = @someValue;";

using (var cmd = new SqlCommand(sql, myDbConnection))
{
    cmd.Parameters.AddWithValue("@someValue", someVariable);
    using (var reader = cmd.ExecuteReader())
    {
        ...
    }
    // Alternatively: object result = cmd.ExecuteScalar();
    // if you are only interested in one value of one row.
}