Getting Started
What do you need to do first?
Before getting started with Nerd Reps, you at least need to know hiragana. You can probably get a fairly solid understanding of hiragana in less than one week.
You might not need to master hiragana because you will be using it so much that your profenciency will improve as you go. So, maybe just get the basics down and move on.
Where to learn?
I recommend Tofugu's Learn Hiragana to get a primer on hiragana.
Do some reps:
Katakana
You need to learn Katakana at some point and it's beneficial to do it early. However, most of the content in nerdreps is based around Hiragana.
--Sprocket