Start with placements you can prove directly from the grid. Each lesson below shows an actual puzzle position, the candidates and the next move.
Every digit from 1 to 9 appears once in each row, column and 3×3 box. Candidate numbers are digits that remain possible in an empty cell after applying those three constraints.
Both patterns are useful on Easy and Medium puzzles. When you stop finding them, revisit your notes after each placement; one elimination can expose the next single.