* The rules of Sudoku are simple: Put the numbers 1 through 9 into the blank spaces in the grid. Every row, every column, and every 3x3 box must contain the digits 1 through 9.
* There is no need to guess. Every Sudoku puzzle can be solved logically. The best way to start is by looking for clues. Most people start by looking for numbers that occur frequently in the initial puzzle. Say you have a lot of 5's in the initial puzzle. Look for the 3x3 box where there is no 5. Can you rule out places where a 5 might go? Look for 5's in other rows and columns that can help you eliminate where the 5 might go in that box. Remember, a 5 can't appear more than once in any 3x3 box, row, or column. If there is a 5 in column's 1 and 2, then there can't be a 5 anywhere else in either of those columns. You know then that whatever leftmost 3x3 box that is missing a 5 must have it go in column 3. If you can eliminate all the possibilities in that box except for 1 square, you've got it down!
* Once you've finished looking at all the 3x3 boxes make sure to fill in the 1-9's in rows and columns that have most of the numbers in them. If there are only two numbers missing in a row or a column you can usually use a process of elimination to figure out where to fill in the last two numbers. Scan the row or column where the missing numbers two are, if you see or eliminate one of possibilities based on the other row or column you can now complete the line you were working on.
* These two steps will solve most basic or easy Sudoku puzzles are all often called solving the lone number or eliminating singles. Difficult puzzles require solvers to look for contingencies in the puzzle. Contingencies usually require a solver to mark-up the puzzle with all the different options in each square. Sometimes there will be no way to eliminate a possibility for a square and you will have to choose between two or more numbers. In these cases it is important to remember where you made the choice to choose a number and backtrack if your original choice was not correct.