Sudoku for beginners comes down to one core rule - fill a 9x9 grid so every row, column and 3x3 box contains the digits 1 through 9 exactly once - plus a handful of simple techniques that let you solve any properly made puzzle through logic alone, without ever needing to guess.
The Rules, in Full
A sudoku grid is 9x9, divided into nine 3x3 boxes. To complete it, every row must contain the digits 1 through 9 with no repeats, every column must do the same, and every 3x3 box must also contain each digit exactly once. That's the entire rule set - there's no arithmetic involved despite the numbers; sudoku is pure logical deduction dressed up in digits. A well-made puzzle starts with enough given digits, called clues, that exactly one solution exists, and every empty cell can eventually be filled using only the three rules above.
Scanning: Your First Technique
Scanning is the simplest starting technique and it's usually enough to make real progress on an easy puzzle. Pick a digit, say 7, and look at each 3x3 box in turn: if a box already has a 7 somewhere in its row or column (even outside the box itself), that eliminates several cells in the box as possible spots for 7. Often this narrows a box down to just one legal cell for that digit, which you can fill in immediately. Repeat with each digit across all nine boxes, and easy-to-medium puzzles will yield a steady stream of confirmed cells.
Naked Singles
A naked single is a cell that has only one possible candidate left, once you account for every digit already present in its row, column and box. If a cell's row, column and box together already contain the digits 1, 2, 3, 4, 5, 6, 7 and 8, that cell must be a 9 - there's no other option, so it's 'naked' in the sense that the answer is fully exposed once you check what's already used. Naked singles are the easiest cells to fill and the technique you'll use constantly, but they only appear once you've filled in enough of the surrounding cells - which is exactly what scanning helps you do.
Hidden Singles
A hidden single is subtler: it's a cell where a digit could technically go, and several other cells in that row, column or box could also technically hold it - except that in one specific unit, say that 3x3 box, only one of those cells can actually hold that digit, because it's blocked everywhere else in the box by that digit already appearing in the corresponding rows or columns. The digit is 'hidden' because the cell doesn't look special at first glance; it only stands out once you check, digit by digit, which cells in a box could still hold each missing number.
Naked Pairs
Once single-cell techniques stop finding new cells, naked pairs are usually the next tool worth learning. If two cells in the same row, column or box both have exactly the same two candidates left - say both cells can only be 4 or 7 - then one of those cells must be 4 and the other must be 7, even if you don't yet know which is which. That means no other cell in that same row, column or box can be a 4 or a 7, letting you eliminate those digits as candidates elsewhere and often trigger a new naked or hidden single.
Using Pencil Marks
Pencil marks - small numbers written in the corner of a cell listing every digit that could still legally go there - are what make naked and hidden singles and pairs actually manageable rather than something you try to track in your head. Fill them in for every empty cell once scanning stops finding easy answers, update them every time you place a new digit, and re-scan for singles and pairs from there. It feels slow the first few times, but it turns a puzzle that seems stuck into one where the next move is usually staring back at you from the pencil marks themselves.
Why Guessing Is Never Actually Required
A properly generated sudoku puzzle is built and verified to have exactly one solution, which means every single cell can be determined through some combination of the techniques above - scanning, singles, pairs and, on harder puzzles, a few more advanced patterns beyond what a beginner needs. If you ever feel like you have to guess, it almost always means there's a technique you haven't tried yet or a pencil mark you forgot to update, not that the puzzle itself demands luck. A sensible learning order is: master scanning and naked singles first, add hidden singles once those feel automatic, then bring in pencil marks and naked pairs for harder puzzles - each layer only becomes necessary once the simpler one runs dry.