Quick answer
Жду с нетерпе́нием!
zhdu s neterpéniyem — literally “I wait with impatience” — instrumental
The options
The instrumental idiom
С нетерпе́нием — “with impatience” — is instrumental after с. The same shape gives с удово́льствием (with pleasure), с ра́достью (with joy) and с трудо́м (with difficulty). A whole family of adverbials.
In writing
In an email, С нетерпе́нием жду ва́шего отве́та — looking forward to your reply — is the standard formula, with the thing awaited in the genitive after ждать. In speech the short Жду! does the same job.
What you hear back
Typical mistakes
Я смотрю́ вперёд к э́тому.
Жду с нетерпе́нием.
A word-for-word translation of “look forward” means nothing in Russian.
Жду ваш отве́т — in formal writing
Жду ва́шего отве́та.
ждать takes the genitive for things awaited.
Check yourself
How does Russian say “I’m looking forward to it”?