Drink water, eat food, don't get hurt.
It's shockingly easy to stay alive without aid from other people when a human reaches adulthood. Humans have done an excellent job of taming the world. There are not a whole lot of seriously dangerous places (except for the humans living there).
If they're the only person left and there are no new dangers, they can simply set themselves up in or near a grocery store and eat non-perishable goods for years to come. If they run out of food or bottled water in one store they can simply go to another one. Canned food can be safe to eat for decades. In that time they can easily find alternate food sources through gardening or scavenging longer lasting packaged food (rice can last more than 30 years).
They can even go down to the local library or bookstore and start reading about various survival techniques, refresh on calculus, or read trashy romance novels out loud for fun.
They are likely to have a more comfortable life than a majority of humans that have ever existed.
To answer you specific questions:
What is worth the effort knowing that any serious injury could be game
over? Building electrical generator? Keeping a vehicle running?
Maintaining a house?
See below for what to learn. You don't need to build a generator, just scavenge a new one if the first one you find breaks. There will be a lot of them around. A similar idea for a vehicle, you'll only need one for the first few years to bring large supplies back to your home base, store a few small mopeds and vehicles indoors for later excursions. You don't need to maintain a house, just live on the main floor of a modern skyscraper. It's not going to collapse anytime soon and should remain in a decent state for many decades.
What resources should be cared for in the first place assuming that I
live in a city?
Food, fuel, hunting supplies, and clean water.
What, if anything, I should learn?
How to hunt and preserve the meat, how to grow your own food, and how to treat wounds and illness.