How do you write 13 in Japanese?

Please keep in mind that 13 is only an unlucky number in Western countries.
The Japanese Word for thirteen is 'juusan', literally 'ten-three', and pronounced 'juu-sahn' - as it's a long 'u', you need to elongate it when you say it.
It is written like this: 十三
Keep in mind that Japanese usually uses Arabic numerals (the numbers we use) when writing horizontally, and kanji (symbols borrowed from Chinese) when writing vertically.

If you cannot see the above symbols, you need to install a Japanese language pack for your computer.