If you are moving to a smaller place, it's quite common sense that you can't bring everything. Some things would have to go and others would have to be kept some place else. To decide which things you can bring you have to know exactly how much space you have in your new place.
Which Furniture to Bring
It's really quite easy to decide. Simply take measurements of your new place. Any furniture that would not fit in the new place would have to either be sold or stored away for the moment.
You can even draw a layout to try out various positioning for the furniture. If an essential type of furniture will not do, you might have to replace it with a smaller one.
Things to Sell and Discard
Some things just lay there without being used for decades. You have to decide now whether you will still get a chance to use it. If not, perhaps it's better to sell or to give it away instead. This will lessen the clutter in your new place and this will also lessen the things you need to pack and bring.
Things to Store
There are also some things that you may not have a place for in your new home yet but you will still use them in the future. Other things have sentimental value. So instead of selling them or giving them away you could keep them in storage facilities until you need them.