1. Most of what you want to know has been covered here and it's pretty good advice.

2. That said, it's not been codified or put into order, so I'll do a tiny list of key/starting points.

3. Remember, every professional philosopher or anyone who as studied the subject at one stage beyond a Reader's Digest summary has his/her pet subject! They will push you in that direction. This is not good for a beginner (but maybe so later).

4. Philosophy has and goes through fashions, avoid obvious ones until you're experienced (stick with traditional stuff to begin with).

5. Try not to study philosophy by yourself. As some others have said, you can skew off course, this may make you set in your ways (which many be difficult if you ever need to go 'mainstream' later).

6. Philosophy has a long and wonderful lineage. Don't short-circuit the old stuff in the hope of bypassing it for newer ideas. The reasons are: (a) that the best books on certain subjects are the oldest ones, (b) many later books will assume you've read and are familiar with the propositions and arguments in these older books (they're longstanding classics full of good stuff that's stood the test of time). In essence, many of these ancient books are prerequisites.

7. Philosophy has many threads, however there are some basic ones that are fundamental. You need to understand these. For example:

(Parts of the list below I've lifted straight from Wiki to save time. However, you don't need all of this stuff listed on this site to begin with (however, I've listed the most essential topics below) https://en.wikibooks.org/wiki/Introduction_to_Philosophy/The...:

7a. Epistemology - The theory of knowledge. You have to lean the 'alphabet' of philosophy before you can do much.

7b. Metaphysics - Wiki defines it as meaning 'after physics'. I prefer the one I was taught which is 'above and beyond physics'. It deals with subjects that cannot be solved rationally by science such as first principles, of being (ontology), what reality is, and such. Even questions such as 'does God exist?'

7c. Logic (formal logic) is essential. This can be a dry and sometimes boring subject but it's absolutely essential if you want to argue, or know the subject of a discussion from its predicate, and best (and most enjoyable) place start is Book 1 of Plato's Republic. No only will you get the structure of formal argument, but this wonderful dialogue between the sophist Thrasymachus and the philosopher Socrates presents one of the most important issues/questions in the world which is What is Justice? Watching Socrates demolish Thrasymachus's 'Every man for himself' argument by formally demolishing every line of it is great stuff (whenever I read it, it sends shivers of excitement down my spine). Remember, this book is about 2.300 years old so it must have something going for it if it's still in philosophy courses! [BTW, probably the best and most well known translation is Benjamin Jowett's version first published in 1881 (there are many copies of it on the net).]

That's only the beginning. In logic, you'll soon progress to formal nomenclature and truth tables and words such as 'vel' and 'existential quantifier' will become familiar to you. Logic is essential as it provide the means to stop you getting sidetracked by junk arguments and sophistry.

7d. Ethics is the study of morality, ethical behaviour, morals, etc., (it's sometimes called moral philosophy). It's the study of right and wrong in human endeavours. In today's world where traditional ethics has gone off the rails, the study of ethics is absolutely essential. The key person to study here is the English philosopher G. E. Moore, and the definitive text is Moore's Principia ethica.

7e. Philosophy of Language. an ancient branch of philosophy which gained prominence in the last century under Wittgenstein. Basically it's concerned with how our languages affect our thought. Classic works include Plato's Cratylus, Locke's Essay Concerning Human Understanding and Wittgenstein's Tractatus Logico-Philosophicus. Another book in this class is A.J. Ayer's Language, Truth, and Logic.

7f. Philosophy of Politics/Government : Closely related to ethics, this is a study of government and nations, particularly how they came about, what makes good governments, what obligations citizens have towards their government, and so on. Classic works include Plato's Republic, Hobbes' Leviathan, Locke's Two Treatises, J.S. Mill's On Liberty and Utilitarianism and Rousseau's Social Contract. Sometimes students skip this one but it's essential to know how our governance came about and the meaning of the Social Contract that exists between the Citizenry and Government.

There's much more, but the ones I've listed above are the basic/fundamental essentials.

As mentioned, don't train yourself. Instead, seek out an old fashioned philosophy school that hasn't been corrupted with too many new ideas. As I said, you need to read works such as those directly above—and I mention them again such as Plato's Republic, Hobbes' Leviathan, Locke's Two Treatises, and J.S. Mill's On Liberty and Utilitarianism and Rousseau's Social Contract (these have been in the philosophy corpus for aeons, as they're very substantial works whose propositions are well known and well argued).

(Others, in the rush, have I missed anything major here?)