[kotonisuru] 日本へ行くことにする I have decided to go to japan
[kotoninaru] 日本へ行くことになった It has been decided I will go to japan
[kotoninatteiru] 日本へ行くことになっている I go to Japan (eg as a rule, once a month)
dc
according to a textbook I once used:
Use にする to indicate a decision made at one's own will; use になる to indicate a decision or change of circumstances that is generated in the course of one's dealings with others.
yookoso
Generally, こと is a nominalizer that makes whatever preceds it into a noun or a nominal. 行くこと would be "the act of going" 食べること would mean the act of eating, etc.