How can I secure my Belkin54g?

The PIN code supplied with the SIM card protects the card against unauthorised use. The PIN2 code supplied with some SIM cards is required to access certain services. If you enter PIN or PIN2 code incorrectly three times in succession, you are asked for the PUK or PUK2 code. If you do not have them, contact your service provider. The module PIN is required to access the information in the security module of your SIM card. The signing PIN may be required for the digital signature. The barring password is required when using the call barring service. To set= how your phone uses the access codes and security settings, select Menu> Settings> Security.
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To secure your wireless network from unauthorized access,please click on this link and follow my instructions :- http://www.fixya.com/support/r3559128-wireless_router_security

Its asking for your wifi security password/passkey. If this is not it then your works network may have other software in place to stop unauthorised access to unauthorised computers.
If you have permission to use the connection then just ask the I.T. manager

By using your wifi security.
Log on to the router and turn on WPA2 security, choose a password, and you should be ok

Check out this link below to download a PDF manual for your phone. Just copy and paste into your browser. I'm not sure what phone you have as the header refers to an LG and the description refers to a Nokia.
If this link below does not help send me the Brand and Model you are asking your question about.

http://nds1.nokia.com/phones/files/guides/Nokia_1600_UG_en.pdf

Access codes from the manual at link above page 12:
• The security code, supplied with the phone, helps to
protect your phone against unauthorised use. The preset
code is 12345.
• The PIN code, supplied with the SIM card, helps to protect
the card against unauthorised use.
• The PIN2 code, supplied with some SIM cards, is required to
access certain services.
• PUK and PUK2 codes may be supplied with the SIM card. If
you enter the PIN or PIN2 code incorrectly three times in
succession, you will be asked for the PUK or PUK2 code. If
you do not have them, contact your local service provider.
Select Menu > Settings > Security settings to set how your
phone uses the access codes and security settings.

Hope this helps, let me know if I can do anything else for you.
JimmyC

do you have the "default" ones? you may need to reset the router

check your router and make sure there is no password in the security tab. You may need to use a wep or wap pass phrase a password to connect.

There is no default WEP / WPA key for belkin54g routers. The default password to access the setup is blank. Try going to 192.168.2.1 in internet explorer (where you would usually type the address of the site in), and see if you can find out the WEP / WPA Key.

There are 2 things you can do. One is to connect the router to the notebook via ethernet cable. You will then be able to enter the router's page by typing 192.168.2.1. The other option is to reset the router to its default factory settings. This is done by pressing the reset button on the back of the router and hold it down for 30 seconds with power still on.

i take it by your comment "unauthorised people been accessing it" that you wish to use the router wirelessly? do you currently have any type of encryption set up on the wireless signal at the moment eg WPA(2)-PSK, WEP, etc?