How do I restrict access on my home network?

I have the occassional family member over and they always want to connect to my wifi. My problem is I can see everything on my drive from any computer that connects to my network. Any ideas?

I guess I should have added that I have 3T WD Live. Also, I have set up password protocols but it would appear that it has no affect on the home network. I did not have to log into anything and all files in public or not are visible at all times on all computers.

Use a router that has Guest Wifi features.