N4
はじめる
To start, To begin to
高校生の時にお金を貯め始めた。
I started saving money when I was in high school.
Explanation
「はじめる」 after the stem of a verb means 'start doing'. 「日本語を勉強しはじめた」 (I started studying Japanese). 「桜が咲きはじめた」 (The cherry blossoms have started to bloom).
The verb before it should be an action or change that lasts for a while.
Mixed up with
「~はじめる」 is a planned or calm start. 「~だす」 is a sudden start: 「雨が降りだした」 (It suddenly began to rain), 「赤ちゃんが泣きだした」 (The baby suddenly burst into tears).
For a planned start, use 「はじめる」: 「明日から勉強しはじめる」, not 「×明日から勉強しだす」.
It does not go with verbs that finish in an instant, such as 「死ぬ」, 「落ちる」, 「着く」.