Both are of utmost importance, but Hiragana is usually more common, so start with it and immediately learn Katakana. Kanji is a gradual process, so don't rush it, learn it little by little as you learn vocabulary and grammar. But mastering both syllabaries is the first step to learn Japanese.

Start with the Hiragana and move to Katakana. You must know both of fluently to read Japanese.

Start with hiragana ;)

Ordinary, people start learning Hiragana. However , it is not wrong way that some people start learning Katakana. This is because both of Hiragana and Karakana are phonogram. At least, I do not recommend to start learning Kanji without the case you have the background at Chinese Character.