はじまります is not the same word as はじまり.
"はじまり" is a noun as written "始まり"
but the part"はじまり" from "はじまります" is a verb as written "始まる”
so you should think " はじまります" is like "始まる”+"ます".
then when verbs get "ます" at the end, very last hiragana in verbs change.
ex. "行く"→"行き+ます" or "履く"→ "履き+ます"