learning hiragana and katakana at the same time is the best ^^ GOOD LUCK!

You need to learn Hiragana first. Otherwise you are not going to be able to read any Japanese, and even not going to be able to read any Kanji. Also do you know how many Kanji are used in Japanese? It's uncountable. It's impossible to learn Kanji first.

I think you should learn hiragana first. Because it is alphabet of Japanese. But Kanji is vocabulary of Japanese besides it can't read if you can't Hiragana. It can be read many ways.

Definitely you should learn all hiragana first.