A doubt about "estas"
In my mother language we have two different words to the verb "to be", "ser" and "estar". Actually it is so in every Romantic language. Esperanto, though, uses always "estas" and considering it's a language with Greek-Latin roots, isn't it supposed to be there a sort of a "ses" too? I'm a beginner, so please correct me if I'm wrong, but isn't the best form "Vi 'ses' bela" intead of "Vi estas bela"?
Actually it is so in every Romantic language.
No, only some use dual copulas, while other Romance languages have only a single copula.
considering it's a language with Greek-Latin roots
Actually, Esperanto stole bits from all over Europe, not only the Romance language branch of Proto-Indo-European.
So, basically, Esperanto has a single copula making for easier usage at the loss of the nuances one can have with the dual-copula systems.
French is a notable example where "ESSE" and "STARE" merged as "être", partly through sound changes that made the two roots sound nearly identical anyway. (You can see, for example, that "tu es" is from Latin ES, a part of ESSE, but the infinitive "être" has a circumflex that hints at a former -s-, indicating that it's from STARE. The past, on the other hand, is étais, été, etc. with é instead of ê: these are from ESSE again.)
That's interesting: I just looked it up and I see that Latin only had one, but the verb "to stand" evolved into a second copula in later Romance languages, only to merge back in in the case of French, as you explain. The other language I know a bit of, Irish, also has two copula but presumably this arose separately, the existential copula in Irish is very odd, it doesn't behave at all like a normal verb.