start, begin

初めて(はじめて)	For the first time ever.

最初(さいしょ)	At the beginning, first of all. $$$

初めに(はじめに)	Pretty much the same as 最初.

開始(かいし)	A formal way to say ‘start’:the commencement.

始める(はじめる)	To start (verb form).