Are there any 'cages' which are single cells? The labels for these squares simply tell you the number inside the square, so that can be a good starting point.
Is there a cage containing three squares in a line? Can you work out the three numbers, even if you don't know their order? How does that help you work out the value of the remaining square in that row or column?
If you have cages of two squares, you may not be able to know for certain what the value of a particular square is, but can you tell what the other two squares in the row or column must be? You will need to use information from other cages to be sure of the values.
If you are stuck, click on the button 'Show me a square I can solve'. A question mark will then appear in a square which is solvable.
If you need to, you can then click on 'Give me a hint about this square', which will suggest how you might go about working out the number in that square.