The Basics:
Before diving into strategy here are basic concepts and rules of a sudoku board: Sudoku grid consists of 81 square tile divided into none columns named with letters a to i and 9 rows from 1 to 9 (kind of like chess but one row/column less) It is also divided into nine 3 by 3 sub grid block. Every row, column, block contains numbers from 1 to 9 without repetition (ony diagonals do not count in classic sudoku.)
Sudoku is a great example of the magic of numbers  the whole board can have only one solution, and for every single tile there is only one possible number to place.
Board scanning
Easy: scanning. in most easy/medium boards you can scan rows, columns and blocks to find the spots where you can already place only one figure:
1. Scan for number repetitions on the table.
You need to find the numbers with the biggest count currently on the table (Most advanced sudoku games can do that for you  just tap the right number  and all occurrences will be highlighted on the table). See image below: we have 1 repeated 4 times, 2: 5 times, and 3: 5 times.
2. Scan in one direction.
You can already place missing number in the bottom right block by simply scanning 3 bottom rows: row 7 and 9 already have 3 and the final row 9 has only 1 empty cell in bottom right corner.
3. Scan in 2 dimensions: both vertically and horizontally
See another example, columns A and B have 3 placed already, and row C has 2 possible placements. In this case you need to scan horizontally. Apparently row 7 already has 3, giving you only one possible opportunities.

Scan for single candidates.
As long as number cannot be repeated neither horizontally, nor vertically, nor i a block. Whenever you spot a number surrounded by another 8, you already got the solution.
See the example above and check what options have you got:
2  in the column & the block
3  in the column
4  possible
5  in the column
6  in the block
7  in the row
8  in the row
9  in the column
At the end you can fill this sudoku cell with only number 4. Profit!
 Single elimination Whenever a row, column or block is missing 23 numbers, you can scan for elimination opportunities. At the example below only 5 and 9 are missing in the column F. However F1 cannot contain as it already exists in the block  therefore only 5 can be placed there, which leaves us with 9 at F6