If you want to make history a serious, life enriching hobby, I suggest multiple levels of reading. Start with the first, and follow your interests down to the next level of detail. It is likely that, as you study history, you'll find something that you are particularly fascinated by - follow that fascination down whatever rabbit hole it leads.

So, to start with, you'll need to figure out what interests you, and the way to do that is to read surveys. Even a general-interest 'introductory' text (if it has any worth as a work of history) is generally relatively focused. For instance, the Oxford History of the US is broken up into ~30-20 year chunks - 1781-1815, 1815-1848, 1848-1865 etc. These are still 'introductions', though, and they are about the broadest scope single-volume histories that are really useful for historical study.

So even before you start reading - what interests you? Is it Greece and Rome? The Middle Ages? American History? Soviet History? Are there topics you are interested in across periods - the history of technology, the history of gender, the history of fashion, art history, military history?

Once you have read a few general works, you should have a more specific idea of where your interests lie. After that you can read specialized books in sub-topics of this field of stufy, and also tackle more technical and specific works in your field of interest - generally these are books on a particular topic in a particular period. Then you can start reading journal articles and primary sources - getting into the original sources and scholarly debates that inform a topic's study. For this, resources like academia.edu are very helpful, and online archives. Depending on your topic, you may find that online museum collections are useful as well. If you get really serious about history as a hobby, you'll probably make friends with academic historians, which is great for both your social life (historians are fun!) and because they'll have access to resources that you don't, like university libraries and agregator services.

To use my own studies as an example, I've been researching armour and weapons for around 15 years. I started off with easily available general introductions - 'Arms and Armour of the Medieval Knight' by Edge and Paddock, and various popular military history books (be careful with these!) I also started informally looking at original art and depictions. Just after college I had a friend who worked at a university library generously offer to borrow 'The Knight and the Blast Furnace' for me. This is probably the most comprehensive academic work on armour (at least from a metallurgical perspective). After that (within the last couple of years) I read as many journal articles and doctoral theses as I could get my hands on, and got more serious about studying originals in a systematic way.

Reading is of course a good option, perhaps look into something you're passionate about and see what's been published recently, it may sound odd but Wikipedia references often show you the most important works in the field. I personally recommend podcasts, I decided a few years ago to learn more about the History of Rome. 'The History of Rome' by Mike Duncan is extremely popular podcast, I loved it personally. So the answer does depend principally in what you want to learn about, whether that's a person, nation, themes etc. But you're likely to find accessible and informative podcasts for most things. Hope this helps.