From easy to hard
Sudoku Strategies
Naked singles
If a cell has only one possible digit, place it. Scan after every placement — one single often creates another.
Hidden singles
A digit may fit in only one cell of a row, column, or box even if that cell still has other candidates. Place it.
Pairs and box-line
If two cells in a unit share the same two candidates, those digits can be removed from the rest of the unit. If a digit in a box is confined to one row, it cannot appear in that row outside the box.