Draw squares, not rectangles! You don't want a grid but a checker board.
You can use nested loops: one looping on the x positions, the inner one looping on the y positions (or the reverse, it doesn't matter).
The tricky part is to ensure the alternate colors... It is even harder with an even number of cell per row!
I give the trick because it seems hard to guess for a beginner that never used a conditional...
You can use:

1.       if (i % 2 == 0) {
2.         if (j % 2 == 0) {
3.           fill(255);
4.         } else {
5.           fill(0);
6.         }
7.       } else {
8.         if (j % 2 != 0) {
9.           fill(255);
10.         } else {
11.           fill(0);
12.         }
13.       }

or:

1.       if ((i % 2 == 0) && (j % 2 == 0) || (i % 2 != 0) && (j % 2 != 0)) {
2.         fill(255);
3.       } else {
4.         fill(0);
5.       }

(less repetitions, but harder to understand...)
See https://forum.processing.org/topic/odd-or-even-number for an explanation on %

[EDIT] An even more advanced, and shortened version:

1.       if ((i % 2 == 0) ^ (j % 2 == 0)) {
2.         fill(255);
3.       } else {
4.         fill(0);
5.       }

This one uses the Xor operator.

Hey phi.lho thank you very much for your help. I figured a way that  i understand how to do it. But, i have no clue how to fill it in like a checkberboard... like make it black and white squares. here's the code I have for the 8x8 grid.



int NumBars = 35;


int cols, rows;

void setup() {
  size(280,280);
 
  // Initialize columns and rows
  cols = width/NumBars;
  rows = height/NumBars;
}

void draw() {
 
  // Begin loop for columns
  for (int i = 0; i < cols; i++) {
    // Begin loop for rows
    for (int j = 0; j < rows; j++) {
     
      // Scaling up to draw a rectangle at (x,y)
      int x = i*NumBars; //  width
      int y = j*NumBars; // height
    
      stroke(0);
      // For every column and row, a rectangle is drawn at an (x,y) location scaled and sized by videoScale.
      rect(x,y,NumBars,NumBars);
    }
  }
}





I just need to know how to fill in every other square but i have no clue how to do that please help :)

see other thread.

continued there

" I just need to know how to fill in every other square"
It was exactly the point of my message... I am so obscure in my explanations?