This is an excellent question.
First, you should know that you can write Japanese ALL in Hiragana. The tricky thing about this, is that your sentences get very long. For example, if I wanted say,
"I eat rice every day."
You could write it this way;
Now, as you can see, this gets a little confusing, as you don't know which are the nouns, particles, or verbs.
Let's write it with kanji:
See how short it gets? The Hiragana characters in there - are either particles (wa/o), prefixes (go) - or the trailing of a verb (~beru).
Almost all nouns have Kanji characters - and so in newspapers and books, you will see that this is written in Kanji. Also, most verbs start with a Kanji - and usually have a Hiragana trailing, such as:
食べる (ta-beru) "to eat"
買う (ka-u) "to buy"
飲む (no-mu) "to drink"
遊ぶ (aso-bu) "to play"
Particles are NEVER written in Kanji - they are ALWAYS in HIRAGANA.
は、が、を、に、 へ、と、 etc.
I hope this helps!