I am planning to share a wireless network DSL line with some neighbors in my building. Is there a chance that someone who is not authorized uses the connection. How to protect the wireless network from intruders and how to find out if someone is logging in illegally??

You should be able to set up some kind of wireless encryption for security, depending on your router and the wireless network adaptors being used. Preferred encryption would be WPA, but WEP might be sufficient. Whoever wants access to your wireless signal would need to have the encryption key (an alphanumeric "password") in order to be able to log on. A smart or persistent hacker might be able to "crack" a WEP key, so WPA would really be preferrable.

1 Corinthians 15:3-4

Let me make sure that I got that right. Those WEP or WPA come with the router itself or I have to get them separately, and where? If so I should look for a router that have such incription schemes right? Any practical suggestions?

The WEP, WPA or WPA2 are common encryptions that are supported by wireless routers, check the features of your wireless router to confirm the supported encryption. Read this wireless network security article to know how to secure wireless network.

Gentleman ...

Wireless encryption is set in your router setup. However, the network cards used by the PCs/laptops must also have the same encryption options, and everyone who wants wireless access thru your router must be set to the same encryption (e.g., all must be using WEP, WPA, or WPA2), using the same encryption key. Not all wireless network cards are capable of the same settings. Older cards may have WEP capability, but not WPA.

1 Corinthians 15:3-4

Ok, thanks guys.