Japanese Writing

Trying to figure out when the Japanese use Hiragana and when they use Katakana. I assume Kanjis are just used whenever a Kanji is available and it's not necessary to make clear the pronunciation.

I've got a japanese mint here and it's got three things written on it. At the top it says "nodoame" in hiragana, then it says "nodoni" in hiragana, and then suddenly switches to katakana and says "sutsukiri".

No clue what any of it means, I've just been learning about the writing system. So what's going on with the switching between hiragana and katakana?

it's like using upper and lower caps.
CLEARS UP your throat.

The スッキリ is just in katakana there for emphasis and impact, kind of like italics or all caps, as stated.

Thanks guys. Here's a slightly more subtle question: how do native Japanese speakers "think about" their two syllabaries?

Do they think of them as two separate sets of glyphs which happen to describe the same sounds, two completely different alphabets; or is it like us with lower-case/upper-case, and they just think of them as one single alphabet wherein there are two different versions of each glyph?

英語の大文字と小文字のように、平仮名とカタカナは一体となって、ひとつの文字体系をつくっている、というのが日本人の感覚です。
（日本語でごめんなさい）