"I have a reservation."
Translation:Yo tengo una reserva.
When the words ends in "a" it's usually feminine, when it's feminine it's "una" when it's masculine it's "un"
I believe tengo refers to yourself and tiene is when you're speaking of other people i.e. yo tengo ella tiene.
Un is masculine for words that end in o and una is for femenine words that end in a
Haven't come across "reserva" meaning anything else but "reserve" until...well, today (7/15/18)! The RAE's online Diccionario de la lengua española (2017) doesn't include "reservation" as another definition. Nevertheless, a generic Google search confirms that nowadays, "reserva" = "reservación." Wish there was a reference out there that records when a new word or new word definition entered the Spanish language or better still, reports the earliest known texts where said word appeared.
Wonder if this is another example of a trend far more common in French, namely "guillotining," where relatively longer words get their final syllables chopped off to make new ones, e.g. "sympa," "fac," "prof," etc.