In The Beginning
To Begin With, To Start With
expression
Remember how 始める means "to begin something"? 始め is its noun version and means "beginning," as in the start of an action or process. に marks a point in time. So 始めに means in the beginning.
始めに can also mean to begin with or to start with when introducing an opening action, like 始めに、ウォーミングアップをしましょう (let's begin with some warm-up exercises).
The reading is kun'yomi (look at all that hiragana!) but that's the reading you learned with the word 始める, so have no fear, you got it!
始めに、プレゼンテーションをしたいと思います。
First, I'd like to make a presentation.
始めにキャベツを切ります。
Cut the cabbage first.
今年の仕事始めにはパスタを食べました。
I ate pasta on my first day of work this year.