I just got a used aquarium from my cousin. It's dry, but there is black stuff on the sides, and I'm not sure how to clean it off. It's completely disassembled, but the parts are inside. Everything except the gravel is open. I can get a picture if needed.
To clean an used aquarium, you need to get some kitchen sponges similar to the ones here.
Be sure they do not contain any soap or detergents. Be sure to use only clean water when cleaning inside the tank.
You need to empty the tank of all of the equipment and gravel before you start cleaning it. Moistening the algae and dirt makes them easier to get off.
The pump and filter need to be cleaned, too. You need to put them in a bucket with water to moisten the dirt and remember to clean it both inside and outside. Use Q-tips for the hard-to-get-to spots.
If the filter sponges is in good shape, you need to clean them by rinsing them and squeezing out the dirt. If you are in doubt, just replace them.
The glass on the heater needs to be cleaned, too. Use a kitchen sponge for this; be careful, it is made of glass.
To clean the gravel/sand, use a strainer and fill it half-full with gravel and rinse it in clean water. It will take some time, but it needs to be done. Even if the gravel is brand new, you need to rinse it to remove the dust.
When all is nice and clean place the tank where you want it and install every thing. Fill with water, add dechlorinator, and start the pump/filter and heater.
It is now time to cycle your tank; take a look at this article on fishlore.com; it is very important to do this before you add any fish. Plants can be added as soon as the water is heated to the temperature you want in your tank.