I would like to know which are the general requirements in order to be admitted at Georgetown. It is enough to have some years experience at a leading law firm, excellent grades at Uni, tax consultancy master, Diploma of research aptitude for doctoral studies? Best regards.

Hard to predict. I'd say you have a fair chance. But this statement is worthless: LL.M admission is a lottery. Only way to know is to actually apply and wait for their decision.
In the absence of any standardized test, the randomness keeps worsening as the amount of applications continues rising steadily every year.
To take the example of Yale and Harvard (2 of the 3 hardest LL.M programs to get into), applicants with no professional experience whatsoever (not even research/teaching assistants at their own university) have been accepted in the past while top of their class Ph.D with over 5 years experience have been rejected.
Several applicants have also reported acceptance at a top 3 law school and rejection from lower ranked top 14 law schools.
So even if your credentials are stellar, there is always a possibility of rejection. In any case, never apply to only one law school.