dame dame dame
! you should say : 日本へ行きます。The "he" is written he but actually read "e" so it is Nihon e ikimasu. e is added when you speak about going to or coming to a place , like 家へ帰ります I come back home . 学校へいきます go to school.
It not so easy to define the actual meaning of ni . As you can use it in speak of your action towards or from someone. like when you say watashi wa tomodachi ni aimasu . I meet a friend . Then you also use it for times and days. like shichi ji ni. mokuyoubi ni. o is quite simple , as you use it for objects like coohee wo nomimasu. spaghetti wo tabemasu . terebi wo mimasu.
good luck v