The verb "å forberede" takes the preposition "til" when you're preparing for something, like an event or an exam. When followed by an infinitive, it takes the preposition "på". It would only take the preposition "for" if you were preparing something for/on behalf of someone.
Prepositions are notoriously tricky, and map poorly from language to language. The English ones still cause me trouble on a daily basis.
As far as I know marriage refers to the state of being married (or, as my dictionary states, a legally or formally recognized union) and not to the act of getting married itself i.e. the wedding. Since a wedding is defined as a marriage ceremony, I guess the translation "We are preparing for the marriage ceremony" should be a valid translation?!?
In the context of this sentence: i presume they are talking about preparing themselves for the wedding ? (the bride putting on her dress and makeup, the groom wondering if he has made a huge mistake etc :P). How does one say "we are making preparations for the wedding", like order the caterer, flowers, priest etc. Thanks.