Securing Wireless Internet used for more than one computer??

It there a way to secure wireless internet that is being used by 5 computers? I live in a house with some other college students and I have reason to be suspicious that someone is picking up my pwords via unsecured wireless. I think this because I received a message on AIM that my sn was signed on in multiple locations when I recently changed my pword (and the pword was only used on this computer- I'm 100% sure). I had gotten a message saying I had an IP address conflict with another computer and I just umed that I got that AIm message because internet probaby disconnected and reconnected when this happened.. But today I was on facebook and it knocked me off suddenly (went to the log in page). This only happens when someone logs onto my account from another location.. again, no one knows my pword! But I dunno how to secure a wireless connection between five computers!