Mexico City is a huge city, for the city alone
one should plan on at lest 3-4 days. It is worth a
trip at any time of the year. One can't see everything
in 3-4 days, there is just so much to see. Just for
the anthropologic museum one needs a full day.
Spring is the best time to visit Mexico in my
opinion. April and May are the hottest month, but
summer is the rain season, and in winter it gets
cold even in Mexico (the city is above 2500 m / 7000
Lonely Planet Mexico
Nearly all large airlines fly to Mexico City. From the
airport one can take a taxi for about 150 Pesos/15 USD
to downtown. For a taxi one first has to by a ticket
at a booth outside the airport building, then one
hands the ticket to the taxi driver. He doesn't get
any money other than a tip.
As in all tourist places one has to be aware of
pickpockets, other than that Mexico City is quite
safe. The 'drug war' happens mostly at the border
region to the USA.
Mexico is tourist oriented, but by far not everybody
speaks English, especially outside the city. The
locals are very friendly and try to be helpful,
therefore one usually ca get by with hand signals if
one doesn't speak Spanish.