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 ;)
