The partial water change should be 30% - 50%. Wait a week, test the water and if all remains at 0, you can add your first fish.
My recommendation is to set up a small QT tank now for the first fish. Yes, this tank will be uncycled so you need to add a small hob
powerfilter, one with a biowheel is a good choice.
Then be prepared to do 20% water changes in the morning and at night for the fiirst week to maintain pristine water conditions in the QT while the bio wheel starts building up a bacteria colony. Keep the fish in QT for 4 weeks to make sure it is healthy and parasite free.
After you transfer the fish to the display tank you can keep the biowheell in the sump if you have one. If not, place it in the tank for a a week before you set up the QT tank for the next fish purchase.
You could of course just maintain the QT tank and get the second fish as soon as the first is moved to the display tank. Then your QT is already cycled and you can just do a 10% weekly pwc