All Japan, All Of Japan
expression
You know 日本 means "Japan." So, now you have the kanji for all added to it, making this word All Japan (as in "All Japan Railways" or something like that).
ぜんにほん, ぜんにっぽん
You know the reading of 日本. Now just add the on'yomi reading of 全 and you should be able to read this word.
全日本の人口は主にアジア人です。
The population of all of Japan is mainly Asian.
全日本の女子バレーボールチームは上手です。
The All Japanese women's volleyball team is good.
全日本の山の中でも、ふじ山は人気です。
Mt. Fuji is popular among all the mountains in Japan.