import java.sql.*;
import javax.swing.*;
import java.awt.*;



Befor you begion initialize your form:
   
   Connection conn = null;
    ResultSet rs = null;
    PreparedStatement pst = null;

in your constructor of your form. In this case login
  public Login() {
        initComponents();
        conn = javaconnect.ConnecrDB();
    }
when button clicked check for login:

private void cmd_loginActionPerformed(java.awt.event.ActionEvent evt) {                                         
        String sql = "select * from users where first_name=? and password=?";
//        String sql="select * from Modules where username=? and password=?";
        try {
           
            pst = conn.prepareStatement(sql);
            pst.setString(1, txt_username.getText());
            pst.setString(2, txt_password.getText());
            rs= pst.executeQuery();
            if (rs.next()) {
                JOptionPane.showMessageDialog(null, "Username and Password are correct");
            }
        } catch (Exception e) {
        JOptionPane.showMessageDialog(null, e.getMessage());
       
        }
    }