Wayne has given you good advice but I'd like to expound on such a little more..
1. Yes, use WPA (or WPA2 if possible) and create a password for it.. Not just any password but something complex with a combination of capital and lower case letters, numbers, and other characters. Use as manner characters as possible.
2. If your router has a firewall, enable it by using the router's settings console.
3. Enable a firewall on each of the computers on the network as well.
4. Run a good, up-to-date antivirus and antispyware on each computer on the network.
5. Use safe surfing practices which will prevent infection of each computer on the network. Those practices include avoiding the use or P2P file sharing services and avoiding unsafe websites.
Hope this helps.