There are things in our life that you should simply leave to a professional, things that you simply should not do yourself. You probably don’t have any business performing your own colonoscopy, your own appendix removal or open-heart surgery. Rocket building and synthesizing your own medicines are simply too difficult to figure out yourself even if you have the parts. But there are things that even doctors would tell you that you should self-examine to find problems early, like cancer exams or home tests for blood pressure/sugar. Cars are the same way.
If you have never worked on a car, you probably shouldn’t be putting in an engine, but that doesn’t mean you can’t use a tool to find out if the check engine light is on for a simple reason. Or if your battery won’t stay charged you shouldn’t see if a new battery fixes the problem before you put in an all new alternator. Some things you can diagnose yourself, some you can fix right there when it breaks, others you can fix yourself after buying the correct parts and getting a tow, and others need a professional hand from start to finish.
A vehicle diagnostic code reader can be plugged in directly to the car’s engine and will spit out codes that pertain to the exact problem. These codes vary and will need to be looked up, but can highlight problems from HVAC (A/C, heating, coolant), engine (not starting, idling, temperature, leaks, emissions), battery (meters, lights, wiring, charging), and brake, steering, transmission and more. These aren’t everything, but can really help cover a lot of your bases. Not everything is a car not turning on or any of its electronics (battery or alternator or wiring issues), a car not turning over (likely starter or engine).
If you would like to have a chance at fixing some simple problems without a tow or need for part ordering, there are some things you should always have: a spare, jack, wrench, jumper cables, duct tape, oil.
Repairs that you can make with parts purchased from local auto stores or online form quite a long list, but this is mainly limited to repairs that you may be able to do yourself. Headlights, brake pads, engine belt, new battery, fluids. You may not be able to do things like alternators, transmissions, engines and things of that nature. Basically, some things are small but fatal to the car running, some non-essential in the immediate like lights, but needed, some of these can be fixed on the side of the road. Others are larger repairs, some can be done at home, but even more need such advanced knowledge and tools that it is not likely to be able to be done at home. Now, if you have a friend who is a mechanic, you may be able to borrow the tools needed to do it properly, but sometimes the cost of the tools needed to fix the car exceeds the cost of taking it to a mechanic. And honestly, that is a long list of things. Even if you had perfect knowledge, it doesn’t make sense to fix yourself if you don’t have a huge number of cars to work on.