CG,
I agree with all that's said above, and add a few suggestions of my own.
1. change the default login and password for your router itself.
Most companies publish this information freely, so a enemy could easily ping your router and then try logging in directly. It goes without saying that once an enemy has control of your router they also have control over everything connected to that router. However that isn't the worst bit of it, the worst that could possibly happen is that they use your internet connection do do something illegal, like massive file sharing of music or movies, underage photographs, things of that nature. Since they are on your network and are using your IP, its possible that you (not them) could be answering the door when the men in black suits come a knocking.
2. Encryption - WPA2-PSK at minimum
When wireless routers first appeared on the scene, a very basic encryption called WEP (Wired Equivalent Protocol) was implemented. However it's sad to say that cracking a WEP is very easy (for examples just google "hacking wep" to see what I mean.) There have been two further advances in wireless encryption since then, WPA and WPA2. WPA2 is about as secure as it gets for a home installation. The downside being that some older networking cards might not be able to support WPA2.
The good news is that WPA2-PSK is very easy to setup, and very easy to distribute the 'pass-phrase' to each item on your network (laptops, Wii, Xbox360, etc.)
I'll leave you with a good further reading page, and of course contact us here with any further questions. The article is a bit in depth, and goes beyond what I've mentioned here, just FYI.
The ABCs of securing your wireless network - Ars Technica
Good Luck!