Question about setting preferred wireless networks. How do I set them?

I just set up a wireless network at my apartment, and I can't figure out how to set it as the default. I also access the school's wireless network regularly but everytime I start up the macbook it tells me that none of my preferred wireless networks could be found and asks if I want to connect to the insecure variation of the schools network. I have the software that gives me automatic access to the secure network installed, so why is it not the default when I'm in range?