I'm going to say something probably unpopular, given the tone of many of the suggestions popping up, but philosophy is one case where I actually prefer to stay away from the so-called "primary sources". They are typically extraordinarily long and verbose for what they have to say, and if the idea some source is groping for is genuinely new, the fact that they are only in the beginning phases of grappling with it tends to show in the presentation. Plus they will tend to use quirky nomenclature that was not necessarily settled on later. If I'm after an actual understanding of philosophy and not merely enjoying some work for its own sake, it's better to read something that has been distilled over the years to its essence and stripped of its ornamentation, so you can contemplate the essence of the idea on its own.

I scare-quoted "primary source" because in a way, the ideas belong to humanity as a whole and the first person to "discover" a particular school of thought is not always its best advocate; it's primary more in the sense merely of first rather than the journalistic sense of authoritative. I would say the journalism or scientific drive for the "primary" sources is much less important here. To me it would be like insisting the only way to tour the Americas is to follow Columbus' precise route and anything else isn't really seeing the Americas.

I say this not because they are worthless or even a bad idea, but that I really do consider them a bad place to start.

Frankly, you could do a lot worse than to just cruise Wikipedia, and drill down in what you find interesting from there. (Don't skip the drilling down. I'm not saying Wikipedia is a full education on the topic. I'm just saying, it's really not a half-bad way to start.)