I would suggest to use a network cable for troubleshooting the problem. (Connect the computer on one of the port numbered from 1 to 4, and plug your modem on the port named Internet)
Once that is done, try to access the router configuration page by accessing the following address
http://192.168.0.1
If you have a connection, you'll get prompted with a username and password. The factory default settings are admin as the username and leave the password blank (or put admin if it doesn't work).
If it still doesn't work, try to do a reset on the router to fall back on the default settings, which you can reconfigure after.
At that point, check if your router have access to the Internet. To do so, click on the Status tab
(example of the page http://support.dlink.com/emulators/di524/st_devic.html )
There should a category WAN, with a line named IP Address. Make sure the address isn't 0.0.0.0, otherwise this is indicating that the network connectivity between the router and the modem is not established.
If that is the case, press DHCP renew to obtain an address from your ISP.
If it still doesn't work, try to turn off / unplug your modem and the router, and turn them on in the following order: modem / router / computer. That should fix the issue.
Once that is done, try to access the Internet from the computer you are troubleshooting from.
If it it working, you should then configure the Wireless security on your router to make sure nobody else except you can access it.
To do so, click on the Home tab, and go in the Wireless section (see example here http://support.dlink.com/emulators/di524/h_wireless.html )
Make sure you use the WPA-PSK security scheme and use a secure password (letter and numbers, lowercase and uppercase and if possible a symbol like @ or ! )
You will have to use this password to connect wirelessly with other computers (most Operating Systems allow you to save it so you won't have to retype it everytime you connect).
I hope my answer was clear enough.