1. Configuration Manager Admin password

Change router Configuration Manager's password form its (default of admin):

Consult your routers manual which will illustrate how you can login to your router and update your administrator password.

2. Use sensible network security

Set network passwords and change them regularly. See your computer's instructions. 

Use good quality antivirus software to protect your computers. Update it regularly with latest virus upgrades

If you enable file sharing on your computer, you should not share your system files and should protect shared folders with a password. See help for your operating system.
If carrying sensitive information (e.g. corporate environment), you should use VPN type (Virtual Private Network) technology to protect your data. Follow guidelines from your company's network administrator.

3. NAT Firewall
Most routers use NAT (Network Address Translation) to assign an address to your ADSL Modem.  Only the NAT address is visible from the outside world and your computers are therefore protected against direct intrusion.

Make sure NAT is enabled consult your router documentation.