I watched a funny philosophy crash course on youtube, and I'm very curious about Plato. As a begginer, where should I start? Original work? Other author's books about his philosophy?
Original. As long as you are interested in a specific author, always the orignial (although for some there might be reading groups/seminars necessary, like Kant, Hegel, Foucault, etc. - but that is not the case for Plato!).
You may miss some subtleties (reading it several times over the course of two weeks helps), but it is still better than reading a possibly wrong interpretation and taking it for granted. That is, mostly an interpretation of a translation, which in itself is an interpretation. You get my point.
Another point: I assume you are not proficient in ancient Greek. Translations vary a lot in their quality. Reading it, you should always stick to sources that offer different possible meanings in their remarks instead of just one interpretation. Greek philosophers often included meaningful wordplays into their writings that may completely twist the meaning if translated badly. Remarks pointing them out from time to time are therefore a good indicator for an acceptable translation.
I agree strongly that Plato should be read directly, but I would suggest there are better and worse reading orders. Plato's work is often divided into three (or more) eras, although this is speculative. The first set is considered to be dialogs he wrote early. These are more open-ended, are considered to be truer to the historical Socrates, and typically end in "aporia" (i.e. with unresolved arguments). The middle era works more explicitly outline Plato's own original metaphysics, still in dialog form, and still for a general audience. The late dialogs are more complex and abstract, more for a specialized audience, and sometimes abandon the dialog format.
I would suggest starting with The Apology to get a look at the historical Socrates. The other early dialogs tend to be easy to read and understand, so those are good for getting your feet wet. I created my own rendition of the Ion, one of Plato's most humorous dialogs, you can read it, if you wish, here (it's a quick read). Then maybe a transitional dialog: I'm personally partial to the Meno, which is the one that first woke me up to Plato's genius. If you can see the hidden layers of meaning in that one it's a good key to understanding all the rest of Plato's work. Finally, you'll want to read the Republic and the Symposium, Plato's most famous writings. They are, without question, two of the great masterpieces of all philosophy, and quite good reads if you find a good translation. You might notice this leaves out all the late dialogs --those are really for hardcore Plato fans only. They are dense and hard to understand, so they probably shouldn't even be approached until you feel you've mastered the earlier ones.
As far as the Theory of the Forms in particular: In my read, the Forms are best seen as one of Plato's most memorable and often used metaphors for a deeper reality that he felt defied any single description. So in once sense, all of Plato's dialogs are about the Forms, in another sense, even those that appear to be about the Forms really aren't. With that said, the combination of Meno and Republic will give you a good overall feel for the Forms as they are most commonly understood.
Plato's Republic. Translated by G. M. A. Grube Revised by C. D. C. Reeve. Hackett Publishing. Note, you will find yourself reading the same paragraph 3 times. This book is akin to taking the Red pill. The cave allegory is legendary.
Here's as good a place to start with Plato as any: The Complete Works of Plato
...some 1800+ pages and best of all: it's FREE (just like the love of wisdom)
normally I'd suggest skipping the introduction but you might find it worth reading some of the "Chronological vs. Thematic Groupings of the Platonic Dialogues" & "Translations" - otherwise, Njoy!
Just wanted to recommend Rebecca Goldstein's Plato at the Googleplex. She is very good at relating Plato's work to real-world situations.