Have set up my d link 2640b router fine,but now i want to stop unautherised use from neighbors etc,how do i secure it

You will need to access the router set up to check the settings of the router.
Is there any other computer connected to the router ?
If not then you will need to connect the computer to the router.
Click Here to know more about how to secure the wireless network

To connect to your wireless router, it needs the router’s wireless settings configured and also needs the wireless cards in all the laptops and or computers to be configured with the router’s wireless settings. Then the laptops and desktop computers can communicate with your wireless router.

To set-up and secure the wireless router from unauthorized access, follow these steps: -
1. Connect the wireless router to your computer with a LAN cable.
2. Start your browser and enter the router's configuration URL IP address.
3. Enter the router setup, login and password
4. Change the admin password
5. Go to the wireless settings and change the default SSID and turn off SSID broadcast.
6. Setup encryption key the strongest is WPA2 (if not then WPA).
7. For further security measures - turn on MAC address filtering and enter the MAC address of the wireless cards on the computer/s you want to connect to this wireless router.
8. Record the settings you have made and keep it in a safe place and Save the settings.
9. Configure the wireless cards on the computer/s with the SSID and encryption settings of the router.
10. Remove the LAN cable between the router and the computer then the computers should be able to connect to the router wirelessly.