allererst
very first
Used to strongly emphasize 'first' in order to express that something is absolutely and without any doubt the first in a sequence.
Das war mein allererstes selbstverdientes Geld.
That was my very first money I earned myself.

very first / first of all
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very first
Used to strongly emphasize 'first' in order to express that something is absolutely and without any doubt the first in a sequence.
Das war mein allererstes selbstverdientes Geld.
That was my very first money I earned myself.

The prefix 'aller-' is used to intensify 'erst' and means 'the absolute first'. You can also use 'aller-' before other superlatives to intensify them, for example 'der allerbeste Freund' (the very best friend), 'die allerschönste Blume' (the very most beautiful flower).
Das war die allerbeste Pizza, die ich je gegessen habe.
That was the very best pizza I have ever eaten.
'Allererst' can stand as an adjective before a noun or as an adverb at the beginning of a sentence. As an adjective it is declined; as an adverb it remains unchanged.
Das war der allererste Schritt auf dem Mond.
That was the very first step on the moon.
As an adverb, 'allererst' (first of all) is often used at the beginning of a speech, presentation, or email. It signals the most important or first point that will be addressed.
Allererst möchte ich mich für Ihre Aufmerksamkeit bedanken, bevor ich auf die Details eingehe.
First of all, I would like to thank you for your attention before I go into the details.
only / not until / first
'erst' and 'allererst' can both have to do with a beginning or a sequence. However, 'erst' is much more general: it can mean 'first', 'not until a certain point in time', or also 'only'. 'allererst' strongly emphasizes that something really comes first or is most important. Learners may confuse the words because 'allererst' looks like a stronger form of 'erst'. That is only true in sequences or when talking about priorities. For times and quantities, you normally say 'erst', not 'allererst'.

Vor dem Training musst du allererst deine Schuhe richtig zubinden.
Before training, the very first thing you have to do is tie your shoes properly.
Der Termin beginnt erst um drei Uhr.
The appointment doesn't start until three o'clock.