Learn French: C'est la ville où nous avons organisé la pétition pour cette initiative. - It is the city where we organized the petition for this initiative.

When identifying or introducing a noun modified by a determiner (like la, un, or mon), you must use c'est. We use il est or elle est only when followed directly by an adjective or an unmodified noun. Since we have la ville, c'est is required.

Does the past participle organisé need an extra -e to agree with la pétition?

No, because the direct object (la pétition) comes after the verb. In the passé composé with avoir, the past participle only agrees if the direct object is placed before the verb. If we had said la pétition que nous avons organisée, then it would need the extra -e.

Is the word où only used as a relative pronoun for places?

It translates to 'where' for physical places, just like it does here referring to la ville. However, in French, où is also used as a relative pronoun for moments in time, meaning 'when'. For example, you would say l'année où for 'the year when'.

If I wanted to just say 'for this one' instead of 'for this initiative', how would I change the sentence?