First off, please see the link in my signature on asking questions the smart way, especially the bits about using proper spelling and grammar. Many of our users do not speak English as a first language, so using SMS-style abbreviations is confusing and just plain lazy.
Secondly, the tutorials are your best friend: "Hello World!" for Solaris OS and Linux (The Java™ Tutorials > Getting Started > The "Hello World!" Application)
Finally, it sounds like you're biting off more than you can chew. Start small. Write some basic programs before trying to create whatever an E post office system is. Try a simple calculator application or a game of pong, and work your way up from there.