I'm completely new to wireshark. I have Ubuntu on a Dell with wireless connection. When I go to Wireshark Capture Option, I cannot select any interface since no interface is listed. What is the problem and how can I fix that?
You probably should check out the article in the wireshark wiki about capture privileges.
If this is a temporary setup you could just start wireshark with sudo. But this isn't really the best approach from a security perspective.
Also see the capturing on un*x section in the FAQ, it has some other things you should be aware of.