You will need some pesos for miscellaneous expenses. Best way to get them is using an ATM machine to withdraw pesos from your checking or savings account. Machines are bilingual (Spanish/English).

Contact your credit card companies before leaving and advise them you will be traveling in Mexico on such-and-such dates. Otherwise there is a decent chance your card will stop working if the credit card company's security system starts detecting unusual activity on your account (i.e. charges originating in Mexico City).

Remember that Mexico City is at high altitude (over 7000') which can take a day or two to get used to if you live at lower altitudes. The air can be dry so you might want to use chapstick and nasal saline solution. Wearers of contact lenses might want to bring along a pair of glasses just in case the dry air and smog proves too irritating. And it is easier to get sunburned at this altitude so if that's a problem for you use some sunscreen.

Have fun!