No!

Moshi moshi!

I was thinking I'd like to learn Yes and how to say agreeable phrases first, but I'm a positive person so I'd rather get the negative out first and then the positive.

So let's learn how to say No!

Chigau is a polite way to say no.
Pronounce like Chig Gow

Chigau masu is a more formal polite no.
Pronounced Chig Gow

Iie is a very distinct "No" and is viewed as a bit harsh
Pronounced EEyah

Iya desu is a more polite I dislike.
Pronounced Eyah des

Dame desu is more like I disapprove.
Pronounced Da May Des

Yamete kudasai is the equivalent to please stop that.
Pronounced Ya Mey Te Koo Da Sai

Alright now we know how to say No!
So next time I'm in Japan and someone offers me some Spinach... (I hate Spinach)
I know just what to say.

Chigau!

Arigato!
Dewa Mata!