Using the pen tool and looking at the reference, start drawing the objects. Try sticking to two-to-three colours per object – this will make getting the right overall colour scheme easier. With symmetrical objects such as a tomato you can draw one side first, duplicate it and place it on the other side. Select the in-between points with the direct selection tool and Option/Ctrl-click-Join to join them.
Continue to draw more objects. You may want to compare them to each other to make sure they are a similar style, both in shape and colour. You could use one of illustrators swatch libraries such as Window>Swatch Libraries>Fruits and vegetables to keep the overall colour consistent.
Once you’ve drawn plenty of objects, try drawing some vernacular shapes. These will help you fill in the spaces in between your objects and can also be a good way to add colours. Try using the grid to keep the curves and proportions the same as the rest of your objects.
Now that you have all the elements you need, you can start to make the pattern. I like to start by making a shape that tiles by itself to use as a guide. Try drawing a square and draw a shape that reaches from the bottom-right corner to the top-left corner. Now use this shape to form four sides of a new shape making sure the bottom-left shape is facing the same way as the top right and the same goes for the bottom right and top left. This will make sure they tile.