Remember don't forget to use namespace, Establish a connection to sql server, use SqlCommand. Paste these code into button_click event.
SqlCommand cmd;
SqlConnection con = new SqlConnection("Data Source=.;initial catalog=YourDataBaseName;uid=sa;pwd=YourPasswordOfSqlserver");
con.Open();
cmd = new SqlCommand("insert EmployeeDetails values '"+
textBox1.Text+"',
'"+textBox2.Text+"',
'"+dateTimePicker1.Text+"',
'"+comboBox1.Text+"',
'"+textBox3.Text+"',
'"+textBox4.Text+"',
'"+textBox5.Text+"',
'"+textBox6.Text+"',
'"+maskedTextBox1.Text+"',
'"+dateTimePicker2.Text+"',
'"+comboBox2.Text+"',
'"+comboBox3.Text+"',
'"+textBox7.Text+"',
'"+maskedTextBox2.Text+"',
'"+textBox8.Text+"','NULL')", con);
cmd.ExecuteNonQuery();
con.Close();