This is about the simplest program you can write in Impromptu.  It loads an instrument and plays a single note.
; make sure that everything is disconnected
; setup simple au graph
; piano -> output
(define piano (au:make-node "aumu" "dls " "appl"))
(au:connect-node piano 0 *au:output-node* 0)
; play note on piano
(play-note (now) piano (random 60 80) 80 (* 1.0 *second*))
The language is called Scheme, which is a member of the Lisp family of languages.  To run this simple program copy the code into your Impromptu editor window.  Then select all the code (cmd-a) and then press the eval button at the top right hand corner of the editor window.  You should have heard a single note play for one second.  You can press eval again to re-evaluate the code.  Notice that random chooses a different pitch each time you evaluate.
Not being one to break from tradition I feel it my duty to also provide a Hello World example.  So here it is.  Delete the code in your Impromptu editor window and copy in the hellow world code.  Once you have copied the code into Impromptu select all (cmd-a) and press eval in the top right hand corner of the editor window.
; disconnected any running graphs
; setup simple au graph
; piano -> output
(define piano (au:make-node "aumu" "dls " "appl"))
(au:connect-node piano 0 *au:output-node* 0)
; hello world as a list of note pitches
; transposed down one octave
(define melody (map (lambda (c)
                       (- (char->integer c) 12))
                    (string->list "Hello World!")))
; Define a recursive function to cycle through the pitches in melod y
(define loop
   (lambda (time pitch-list)
      (cond ((null? pitch-list) (print 'done))
            (else (play-note time piano (car pitch-list) 80 10000)
                  (loop (+ time (* *second* 0.5))
                        (cdr pitch-list))))))
; start playing melody
(loop (now) melody)
OK, so it turns out that Hello World doesn’t make such a good musical example :)
I want to point out one very important point!  Notice that if you re-evaluate the whole text buffer during playback (i.e. while the melody is playing) everything stops and starts again from the beginning.  Selecting a whole buffer and pressing evaluate is NOT the way to use Impromptu (I know I told you to do it :).
Place the cursor just inside the (loop (now) melody) expression so that (loop (now) melody) highlights in a light-blue colour (i.e. not the pink selection colour).  Press the eval button, and press it again, and again, and again.  
Impromptu is dynamic and interactive and was developed for use in live performance.  The Impromptu 101 tutorial will give you a much better idea of how you can build and evaluate code in a more Impromptu friendly manner.
Let’s just finish off with a quick graphics example to see if we can make hello world perform any better :)  Again, copy the code into Impromptu (deleting the old hello world code in the process) then select-all and eval (i know I told you not to do this but ... what can I say ... everything will become clear in time).
(define canvas (gfx:make-canvas 800 600))
(define text-style (gfx:make-text-style "Times-Roman"
                                      (list 1.0 1.0 1.0 0.25)))
(define text-path (gfx:make-path))
(gfx:set-start-point text-path 150 250)
(gfx:add-text text-path text-style "Hello World")
; start with a clean cavas
(gfx:clear-canvas (now) canvas '(0.1 0.1 0.27 1))
(define rotate
   (lambda (time path)
      ; wipe out a bit of the previous drawing
      (gfx:clear-canvas time canvas '(0.1 0.1 0.27 0.025))
      ; display the text-path
      (gfx:draw-path (+ time 1) canvas path
                 '(0.5 0.7 0.9 0.3)
                 '(0.0 0.9 0.2 0.1) 4)
      ; shift the text-path rotation one degree
      (gfx:rotate-path (now) path 1 300 253 *degrees*)
      (callback (+ time 1000) 'rotate (+ time 3000) path)))
; start
(rotate (now) text-path 0.8)
Now just to prove a point.  While hello world is spinning around try typing (loop (now) melody) at the bottom of the editor window.  Place the cursor inside (loop (now) melody) and evaluate.  Hey presto, melody and spinning graphics.  The important point to remember is that all your previous evaluations are still active and will be active until Impromptu is restarted.  So, something to keep in mind, if things are behaving weirdly it may be because of something you evaluated earlier.  If in doubt, restart Impromptu.
Check out some other Impromptu tutorials or some examples , or take a browse through Impromptu function index .  
Have Fun!
Your first program