Sticky launcher should be your main weapon as a demo and works great if you just spam it. Nade launcher is still useful sometimes. Of course, you'll need a hat if you're gonna play.
I'd be impressed if you somehow got a hat at such an early time. 'course, one hat is never enough. You too, shall eventually succumb to the fever of this game.
This site gives a somewhat accurate guide for trading, use it if you are uncertain of some things. http://tf2spreadsheet.blogspot.com/
I couldn't ****in' get outta Artifact Assault lobbies today... Every time I auto-switched lobbies if everyone left, except maybe once, it threw me into a random AA match. Every other time I joined a Chest Capture or Escort lobby, enough people would bloody vote it into an Artifact Assault lobby...