Bibliothecam visitare volumus, sed inaccessa est.
How does this suddenly become we want? Volumen meant books or scrolls how does Volumus suddenly become we want?
volumen and volumus are unrelated. volumus is one of the forms of volo, velle.
Ah, whence "volunteer/volition".
How do I say "We want to visit OUR library"?
Adding nostram would do what you want.
The subject pronoun (in this case "nos") is optional.
(Nos) bibliothecam visitare volumus
We want to visit a/the library.
(Nos) bibliothecam nostram visitare volumus
We want to visit our library.