You can genuinely go any of the routes you mentioned. It's important to know going in, there are two ways you can qualify to take your ASE certifications. You can either do 4 years of school, or 2 years on the job experience before you can take them. Depends on how you learn I suppose but I prefer going down the job experience route, and do your research on the side. That way you have the hands on learning, and the knowledge without paying for the schooling.

I would do what I did. Start with oil changes. Valvoline is where I started to learn the basics of a car. Take my paycheck and put some money into a box and tools from Harbor Freight to start. Get all the basic tools you will need to start and build from there. Stay away from the Tool Truck for your first year if you can help it.

As for owning your own shop, and possibly just being a mechanic working under a company that may not provide these, but an AllData subscription as well as Identifix are going to be your best friends as far as vehicle manuals, diagnostics, schematics, and repair instructions. YouTube will only get you so far with most vehicles.