security mecanism for small app

[Rui Machado <l16338 () alunos uevora pt>]

Hi list,

I'm developing a small client-server app (in Java) to introduce myself 
to security and security mecanisms.
The policy envolves:
-the server only accepts requests from authenticated clients
-the client demands server authentication before sending data to it
-the data must be ofuscated
-there will be daily reports, that must be signed

Since I'm a newbie to this, I wanted to hear your opinions on the best 
security techniques to achieve the best solution and maybe discuss all 
the alternatives so that I can learn as much as possible.
Thanks in advance and excuse my English :)

--rui--