How do you write i am 10 years old in japenese ( hiragana )

Just to add a couple of remarks to Konrad509's answer.

Yes, one of the right answers is:
       
        わたしはじゅうさいです。
      [watashi ha juu-sai desu.]

In kanji it looks like this: 
       
       　　私は十歳です。  
or
　　　　　私は十才です。

If you wanted to customize your sentence a little bit then I would suggest changing the first person pronoun to something else. Therefore, if you're a girl, you might to use あたし [atashi] instead of わたし (私) [watashi]; if you're a teenage boy who wants to sound cool on the other hand, you might want to use おれ (俺) [ore]. 

Also you might want to add よ [yo] at the end of the whole sentence to sound more amiably. 

I hope that it clears everything up for you. If you had more questions, you can just write to me personally ;)

Cheers!