みんなの日本語 中級I - Lesson 3 says that V (dictionary form) ことにする is used to express the decision to do something. If I say
私は日本へ行くことにしました。
this can be translated as “I decided to go to Japan”. This is perfectly clear to me. However, what is the correct translation for
私は日本へ行くことにします。
? Is it “I will decide to go to Japan”, “I decide to go to Japan”, or “I decided to go to Japan”? Can you explain to me when to use する in non-past tense?