First tip:  when posting questions here use a meaningful subject that's related to what you're asking, rather than an admission of your newbie status ;)
Next up: post some code.  It's hard to know where you're going wrong if we can't see what you've done.
Final tip:  whilst there are definitely elements in common between actionscript and Processing there are some fundamental differences you have to get used to; particularly when you're starting out.  For example, whilst it's pretty easy to set up button behaviour in Flash, with over/hit/release states etc., it's not quite as simple in Processing and you're likely to have to delve into OOP; which you may or may not be entirely familiar with depending on whether you've been working with AS3?  So don't run before you can walk.

So; post some code and I'm sure you'll get some help ;)

P.S.  Bonus tip: when you've sat staring at the screen for hours on end and still haven't come up with a solution... take a break!

1. //image array
   PImage [] images = new PImage [10];
2. //define my main images used
   PImage bgLedge;
   PImage lmPot;
   PImage lmTiny;
3. //invisible button coords
   float x = 20;
   float y = 340;
   float w = 30;
   float h = 30;
4. //main bg areas
   void setup() {
     size(400,400);
     stroke(0);
     noFill();
     frameRate(30);
     //background image
     bgLedge = loadImage("windowLedge.jpg");
     //loading numbered image files
     for (int i = 0; i <images.length; i++) {
       images[i] = loadImage("btnLemon" + i + ".png");
     }
   }
   void draw() {
     background(bgLedge);
     lmPot = loadImage("tree.png");
     image(lmPot, 240, -5);
5.   //array display
   image(images[0], -30, 305);
   image(images[1], 10, 305);
   image(images[2], 50, 305);
   image(images[3], 90, 305);
   image(images[4], 130, 305);
   image(images[5], 170, 305);
   image(images[6], 210, 305);
   image(images[7], 250, 305);
   image(images[8], 290, 305);
   image(images[9], 330, 305);
6. 
     //empty buttons
     stroke(0,0);
     fill(0);
     if(mousePressed) {
       if(mouseX>x && mouseX <x+w && mouseY>y && mouseY <y+h) {
         println("ok so at least my button is recognised");
         stroke(0,0);
         fill(0);
         //
       }
     }
   }
   

First advice: put the tree.png image loading in setup() too.
Next, to display the right amount of lemons, you loop the required number of times over the images array. You might need also arrays of coordinates to make it work. Or better, an array of PVectors, but it is slightly more advanced.
You might want to find which "button" is pressed by looping (again) on the check, something like:

1.   if(mousePressed) {
2.     if (mouseY > y && mouseY < y+h) {
3.       for (int i = 0; i < images.length; i++) {
4.         if (mouseX > x + i * w && mouseX < x + (i + 1) * w) { // Might want a margin between buttons
5.           println("ok so at least my button is recognised");
6.           clickedButton = i;
7.           break; // No need to check next buttons
8.         }
9.       }
10.     }
11.   }

I'd guess you're not quite ready to dive into OOP, but it would make this a lot simpler (once you've overcome the inital hurdles).

Still, since your buttons are evenly distributed they can be positioned using a for loop (like the one you use to load the images).  The formula required is simple enough:

for(int i=0; i<9; i++) {
  int imgX = offsetFromLeft + i * (w+gapBetweenButtons);
}

You could even generate a test for each button using a for loop and use the value of i to determine the action to take.  See switch() in the Reference for an elegant way to achieve this...

1. //image array
   PImage [] images = new PImage [10];
2. 
   //define my main images used
   PImage bgLedge;
   PImage lmPot;
   PImage tenP;
   PImage tweP;
3. 
   //invisible button coords
   float x = 10;
   float y = 340;
   float w = 30;
   float h = 30;
4. //main bg areas
   void setup() {
     size(400,400);
     stroke(0);
     noFill();
     frameRate(30);
     //background image
     bgLedge = loadImage("windowLedge.jpg");
     //loading numbered image files
     for (int i = 0; i <images.length; i++) {
       images[i] = loadImage("btnLemon" + i + ".png");
     }
   }
   void draw() {
     background(bgLedge);
     lmPot = loadImage("tree.png");
     image(lmPot, 240, -5);
5.   //array display
     image(images[0], -30, 305);
     image(images[1], 10, 305);
     image(images[2], 50, 305);
     image(images[3], 90, 305);
     image(images[4], 130, 305);
     image(images[5], 170, 305);
     image(images[6], 210, 305);
     image(images[7], 250, 305);
     image(images[8], 290, 305);
     image(images[9], 330, 305);
6.   //empty buttons
     stroke(0,0);
     fill(0);
     if(mousePressed) {
       int x = mouseX / 40;
       if(mouseY>y && mouseY <y+h) {
         if (x == 0) {
           tenP = loadImage ("tenP.png");
           image(tenP, 240, -2);
         }
         else if (x == 1) {
           // Do 20
            tweP = loadImage ("tweP.png");
           image(tweP, 245, 20);
         }
         else if (x == 2) {
           // Do 30
         }
         // and so on
       }
     }
   }