If its the G604T device, it should already have the ability to serve wireless connections. Your laptop would need to have a wireless adapter of some kind, most of the newer laptops do have them so it isn't a cause for concern but the easiest way is to check in your network connections folder for a wireless network connection (start -> control panel -> network connections). If it doesn't have a wireless adapter installed check your laptop specs to make sure it does, otherwise you will need to look at a wireless adapter which depending on your needs goes from $50 upwards (USB adapter).... you can purchase these from Telecom or most retailers. Because this wireless modem has the nessecary bits for a wireless network, once set up the only cables you need to have plugged in are the power and ADSL (to the phoneline) cable.
Going onto the set up itself:
If it is the G604T device, connect via ethernet (blue cable) and go into the admin page (Internet browser -> 10.1.1.1, username: admin, password: admin).
Click on the Wireless button on the left hand side.
Check the settings but if there is only one device probably easier to resetup wireless network.
Ensure that Enable AP is ticked.
Ensure that you have a SSID (preferrably not Dlink or Dlink_wireless)
Ensure that you have a security level enabled (WPA preferred). Would recommend setting it to WPA if it isn't already (set to WEP if you have devices that don't support WPA)
If you have WPA enabled, make sure you select PSK-STRING, not 802.1X. This is sometimes set by default (to 802.1X with no server settings) and will cause problems. Setting it to PSK-STRING makes your life easier. Type the password you want (>8 characters) into the box.
Click Apply. Double check the SSID box to ensure that has what you want as sometimes when changing the security setting box it doesn't save any changes.
Goto the tools tab at the top, click it. Then click the system button on the left handside. Click Save and Reboot.
Wait for system to come back online and check the WLAN light. If its on, try connecting via the laptop... if its off - go back into the admin page and click restart AP.
On your computer (XP SP2 makes it easier for wireless), goto start -> Control panel -> Network Connections -> Right click Wireless network connection -> Select view avaiable networks -> Select your network and click connect -> Enter your password in the password box + confirm box.