Answer

I didn’t decide, and I’m not a rabbi in any formal sense. I didn’t take any rabbinical certification exams, and I wasn’t ordained by anyone.
You should know that the title “rabbi” nowadays doesn’t represent anything formal in the halakhic sense. It is a legal regulation of the state, which recognizes the Chief Rabbinate’s exams as an academic-equivalent credential that allows a person to serve in various positions, and that’s all. It has no halakhic, Torah, or other significance. Whoever people call a rabbi is a rabbi, so this is a decision made by the public, not by the person himself. Most of the great rabbis were not formally ordained as rabbis—some by their teachers, and some, I would guess, not even that.