how can i secure our existing wireless LAN from others within our neighborhood? Coz we are having a wireless router with internet connection and i found out that anybody ca access it.....tnx

a LAN is a Local Area Network: which only has a limited range, and i would guess only your immediate neighbours are able to access it.
Simply use your router manual to find the address to connect to then you can look for the WEP key section, by adding this key it helps protect your wireless connection.
Any computer connecting wirelessly will have to update it's settings to include this key, but since you know what it is.. it is not that much work.

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Enable WEP Encryption ,
Make a strong alphanumeric key so that no one is able to guess it or crack it quick .

Read the manual that came with your wifi router in order to know how to setup the Encryption .

Another option is to enable MAC Address filtering. Every network card or device has a unique MAC address associated with it, which can be added to a table of "safe" or "approved" addresses on your router via the Administration panel.

Only devices whose MAC address is in this list are able to access the router, Internet and so forth. This method is very secure and ideal if only a limited number of machines will be used on the network. However, if friends or family are likely to come round frequently with laptops and want access to the Internet, it can get a little tedious having to retrieve their MAC address and add it to the router.

To find your MAC address, either look on the network card (Also often found printed on the bottom of laptops on a sticker) or go Start - Run - CMD and press Enter. Type ipconfig /all and it will appear as "Physical Address".