Hola,
buenas.
Tengo
una
reservación
para
esta
noche.
OH-lah, BWEH-nahs. TEHN-goh OO-nah reh-ser-vah-SYOHN PAH-rah EHS-tah NOH-cheh.
Hi, hello. I have a reservation for tonight.
"Buenas" by itself is an informal, all-purpose greeting that works any time of day. Very common in casual settings like hostels.