All you have to do is make sure each row column, and each of the 3 x 3 blocks marked out by the black lines has one of each number (or fruit, etc).
There are two basic ways you can fill in a cell:
The Pincer Movement
In this method, we look for cells where the number has run out of other places to hide.
In the image on the left, you will see that two of the three 3x3 blocks on the right already have a two, so we know in which column the two can't be for the bottom 3x3 block.
It could be above or below the three, except that there's already a two in the bottom row, so it can't be there either. The only place the two can be is the cell coloured in solid green.
The Last Man Standing
Look for a square that is surrounded by a variety of numbers on the same row, column, or 3x3 block.
We know that any given square has to contain a number from 1-9. So let's work out what the light green square can't be. It can't be an 8, 9, or 1, because they are in the same 3x3 block already. It can't be a 6, 5, or 3, because they are already in that column. Neither can it be a 2 or a 4 (we've already eliminated 8) because they are in the same row.
In order to squeeze this into my tiny brain, I normally go in order - look for a 1, then a 2, then a 3, etc. In this case, only a 7 is not already placed on a square, so the light green square must be a 7.
Is that it?
Well, not really, but it will get you started. You can solve any sudoku here with those two methods, and if you get stuck, you can click the button, and it will tell you where you can go, and which method you need to use.