。
Do you have a reserved seat?
Literal
Reserved-seat [subject-が] exist-[polite-ます]-question?
Two quick things: 予約席 is a noun compound meaning 'reserved seat' (予約 + 席), and ありますか uses ある ('to exist') for inanimate things — 'is there a / do you have a.' Japanese doesn't distinguish 'is there' and 'do you have' here; context decides.