Assuming you are connected through the router now, click the link below
http://www.routerlogin.net
Login when prompted with the following details
username: admin
Password: password
When loged in, click on the wireless settings link on the left hand side, then select "WPA-PSK (TKIP) + WPA2-PSK (AES)" from the security options and enter a password in the passphrase box, click apply.
Now when you try to connect to your network you'll be prompted for a password, enter the one you just entered in the passphrase box and you are secured.
See here for further info
ftp://downloads.netgear.com/files/WNR3500L_IG
PM_Combo_04Aug09.pdf