What is the formal of :
Takai Biru (tall building) ----------> ?
Biru wa takai (the building is tall) -> Biru wa takai desu.
I know the formal version of takai is takai desu but how the use it in the first example?
As the person said above: Takai biru desu.
Keep in mind, that in Japanese, the part that shows formality is the predicate (or the copula connected to the predicate).
"Itta koto ga arimasen" is the only correct way to say "I have never been there" eventhough "itta" is the casual past time. But, similarly to an adjective, only the casual form can work as a clause-verb/attribtute before a noun; the formal form can't. Expect a few conjunctions, the formal form is rarely found in the middle of the sentence or in a subordinate clause but the final predicate (if it is formal) makes the whole sentence formal.
In your sentence, takai is an attribute and as such, you will use its casual form.
To make the sentence formal, you need to use a formal predicate, for that sake you have to use the formal copula as the predicate in itself can't express formality. Hence desu.