An important fact that you will need to realise about sandpaper is that it is rated by grit. The lower the grit number - fewer, but larger, bits of sand will be present on the sandpaper. The higher the grit number - more, but smaller, bits of sand will be present on the sandpaper. Since the task at hand is to sand wood furniture, it would be best to choose sandpaper with 220 or more grit rating.
When starting a furniture sanding project, it is best to use a coarser paper and then eventually switch to a higher grit as imperfections are removed. For an almost perfectly smooth piece of furniture, you would want to end the project with 300-or-higher-grit sandpaper. However, for starting the project, you might want to use sandpaper with less than 150-grit.
When you start, the surface of the piece of furniture being sanded will be quite uneven. To get a feel of the unevenness of the surface, wipe your hand across the surface. Be sure to sand the piece of furniture with appropriate sandpaper until it feels uniformly smooth to the touch.
Before you begin sanding, it would be best to perform a visual inspection. If the piece of furniture you are planning to sand is laminated, hand sanding would probably be the best option. This is because using a power sander would take off wood too quickly and would thus ruin the piece of furniture being sanded. On the other hand, it would be safe to use a power sander for sanding a solid piece of wood.
To prevent too much wood from coming off while you are using a power sander, be sure to use finer grit paper (220 or more).
Irrespective of how you began sanding the piece of furniture, you will want to finish the task by hand. This will make it easier for you to take into account any imperfections that might remain, and thus remove them.