I need a translation please
 

A friend of mine choose the code name Ryujin and I was wondering if possible could someone tell me how to write it in kanji form.

You seem to have no idea how Japanese names are formed.

If the person who named himself "Ryujin" doesn't know how it's written in kanji, then no one else would, period. We can't go the other way around.

He probably just wants to use the kanji for "dragon king", which is one possible reading of the kanji for Ryujin. If that's the case, you can write it as follows:

竜神

Thank you.