You can find different types of decorations in your Shop.
When you have purchased a decoration, you can place it on areas of your island that have been unlocked. When you are about to place a decoration, you will see a green or red coloured border around it. If you see a red coloured border, it means that you are not allowed to place the decoration there as there is another decoration already placed in that area.
If you see a green border, it means you can place the decoration there.
Tap and hold the decoration you want to move, then drag it in place. Once you are happy with where you are putting the decoration, please tap on the "tick" to confirm.
For Computer Users:
You can move items by clicking on the Tool Icon (mouse pointer), then on the Move Icon (4-way arrow), after which you can click on the item you want to move.