Each Sudoku has a unique solution that can be reached logically without guessing. Enter digits from 1 to 9 into the blank spaces.
Every row must contain one of each digit. So must every column, as must every 3x3 square.
Try the interactive Sudoku tutorials, too.
Here is a link to FREE Sudoku puzzles which range from incredibly simple to notoriously evil. Puzzles can be played online or printed for later use: