how do you say 'I'm not scared' or 'I'm not afraid' in Japanese?

"kowai janai" it's incorrect.

the correct form is "kowaku nai" 怖くない.

The full sentence translates into "watashi ha kowakunai" 私は怖くない

But often the subject is left off because it's understood from the context, so "kowaku nai" alone can be used.

You would say : watashi wa kowaikunai desu.

(pronounced koh-why-koo-nigh)

If you just say not scared, it may sound like you are saying an object is not scary, so unless the person you are talking to knows what your on about, you should use watashi.

scared by itself is simply kowaii, but to make it a negative adjective, you drop an i and add kunai.

make sure you don;t gat confused with kuwaii, which means cute, because I have made that mistake before and you look rather foolish.

hope this helped, good luck!

Watashi wa kowaku arimasen.

Watashi wa osoretemasen.

These examples are written politely.

Dommo ori gatto Mr. Robotto?

kowai janai