erstmal
first
Means 'zuerst' (first) or 'zunächst' (initially) and indicates an order of actions.
Lass uns erstmal essen, bevor wir den Film ansehen.
Let's eat first before we watch the film.

first / for now
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first
Means 'zuerst' (first) or 'zunächst' (initially) and indicates an order of actions.
Lass uns erstmal essen, bevor wir den Film ansehen.
Let's eat first before we watch the film.

'Erstmal' is the commonly used colloquial short form of 'erst einmal' (first / for now). In spoken language and informal texts, 'erstmal' is very widespread.
Ich muss erstmal meine E-Mails checken.
I first need to check my emails.
In the meaning of 'zuerst' (first), 'erstmal' structures a sequence of actions. It often stands in the middle of the sentence, after the conjugated verb, to mark the first step in a chain.
Wir gehen erstmal einkaufen und kochen dann gemeinsam.
We'll go shopping first and then cook together.
'Erstmal' can also mean 'for the moment' or 'for the time being'. It expresses that a situation or action is finished or sufficient for the moment.
Für heute haben wir genug gearbeitet, das war's erstmal.
We've worked enough for today; that's it for now.
only / not until / first
'erst' and 'erstmal' can both come at the beginning of an action and show a sequence. 'erst' sounds more neutral and clearly says: this happens first, then something else comes after it. 'erstmal' is especially common in spoken language. It can mean 'first', but also 'for the moment' or 'for now'. Because of that, 'erstmal' can sound softer: you are not deciding everything finally yet. The words can be confused because both have to do with a first step. If you mean a clear sequence, 'erst' works well; if you mean a provisional first step, 'erstmal' is often better.

Vor dem Termin haben wir erstmal alle Fragen gesammelt.
Before the appointment, we first collected all the questions.
Er liest erst den Text, bevor er die Aufgaben macht.
He reads the text first before doing the exercises.