You need to log in to the router and enable the wireless security setting. Check the manual that came with your router for exact details of how to do this - you usually do it by through your browser by opening the address 192.168.1.1. You may need a username and password - admin seems to be a popular choice among router manufacturers, though these are usually changeable, and it is recommended that you do so.
Generally you'll have a couple of options to choose from: WEP, WPA, & WPA2. There are a couple of variants of WPA - one is for designed for people who have a RADIUS server that stores the passkeys, and WPA-Personal or WPA-PSK (Pre-Shared Key).
WEP and WPA represent two different types of encryption, with WPA being the newer and cryptographically stronger of the two.
Most newer wireless cards are compatible with WPA2-PSK, in which case you need to generate a 26 digit hexadecimal key. This key will be required for any wireless device to connect to your router, so make a note of it. Some routers will automatically generate the key for you, others will require you to type it in.
When your computer tries to connect to the router you will be prompted to enter the key. If you cannot connect to your router wirelessly using WPA2-PSK, try WPA-PSK, and then WEP.
Using wireless security will ensure that all your communications with the router will be encrypted, and that your router cannot be used by 3rd-parties. The BSSID will still be visible to everyone however, though most routers allow you to disable BSSID broadcasting.
Wireless security will NOT enable extra security between the router and your
IBB aerial as that is a wired link, or between the
IBB high-site and your aerial.
Hope that helps,
Paul
Wireless secuity - WPE 
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