Almost everything in your house has a manual. The furnace, the ac, the water heater, the water softener, the coffee maker, the fridge… they all have manuals. If the people before you weren’t responsible and you don’t have a packet of manuals somewhere, go through everything and download them. They all say exactly how to do maintenance for each thing, and how often.

Other than that it’s mostly looking around and making sure nothing is actively being eaten. Take a flashlight and look around in the attic and basement or crawl space or whatever your can’t normally see and make sure things aren’t moldy or rotting. If you catch things earlier it’s always cheaper and easier.