21 Dec at first vs firstly/first of all
At first I didn´t realise that I had forgotten my bag. I only became aware of it when I had to pay at the restaurant.
At first means at the start or initially.
Firstly, First of all, First
First of all, I´d like to thank Paul Smith for inviting me to give this speech…
First/First of all/Firstly means to begin with and it´s used to enumerate points in a speech or piece of writing.