The rules are simple, you must enter the numbers 1-9 in each Row, Column and square without repeating the same number in either...
Take a look below at a blank Sudoku puzzle and the correct solution next to it:
Now take a look at the same puzzle with the rows highlighted, notice that none of the numbers repeat within their individual rows:
Now take a look at the same puzzle with the columns highlighted, notice that none of the numbers repeat within their individual columns:
Now take a look at the same puzzle with the squares highlighted, notice that none of the numbers repeat within their individual squares:
That all looks perfectly easy, so where is the challenge? Well... you cannot repeat the numbers 1-9 in any row, column or square, you also cannot place the numbers in fashion that would force you to repeat that number in any other part of the puzzle
The video below will walk you through the beginners guide to Sudoku, once you understand the basics you'll be addicted!