Select the plane you want as the default and start a flight.  Press ALT when the simulation starts and go to "Flights" on the menu bar.  Select "Save Flight" and enter some name in the window that comes up.  Check "Make default flight" and save it!  Now when you start up, that aircraft and the field you flew at will be the first things up.

You can also set other things this way, like where you are at the airport, what is on or off in the aircraft and other stuff.  Just set everything up the way you want it when you start up and save it as above.  It will stay like that until you change it, even if you change aircraft or airports.  DON'T SAVE WITH THE AUTOPILOT ON!  This is important.  If you do, the autopilot will still be on when you get back into the sim, and if you don't turn it off, it throws everything out of whack on takeoff.