。
I'd like to make a reservation.
Literal
Reserve-want-[explanatory-んですが].
A textbook polite opener: ~たいんですが is the casual-spoken contraction of ~たいのですが (の → ん). Extremely common in everyday polite speech, especially when approaching a counter or service desk — the trailing が leaves the request hanging, inviting the listener to respond. A good minimal example of how ん-contraction pervades polite register.