I would like to know what information is public and can be intercepted in a non-open, but unsecured wireless network. Moreover, is there anything I can do to make it more "secure", other than using https connection whenever possible.

In more details, I recently discovered (with surprise) that the wireless network in my school is actually unsecured. Although not everyone can connect to it (you need a student ID), I am told that certain softwares like Wireshark would be able to intercept the data. Since I have been using the network for all private purposes (email, facebook etc), I do feel quite insecure now and would like to understand the situation a bit better.

I installed Wireshark and tried to play with it but all I can see are something alien to me. In any case, all I see seems to come directly/indirectly from my IP address, and I have long thought that usually different computers in the same wireless network would be assigned different addresses. Am I wrong? If not, then I feel very confused about what information is actually being captured (potentially by other users in the network, since I don't think I could capture activities of others in the same network anyway), and whether it's safe to use the network at all. (Gambling on others in the same network showing good behaviour is apparently not an option.)

Thank you.