vorweg
beforehand
Before something else happens or is said; in advance.
Um es gleich vorweg zu nehmen: Ich bin mit dem Vorschlag nicht einverstanden.
To say it right at the outset: I don't agree with the proposal.

beforehand / ahead
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beforehand
Before something else happens or is said; in advance.
Um es gleich vorweg zu nehmen: Ich bin mit dem Vorschlag nicht einverstanden.
To say it right at the outset: I don't agree with the proposal.

The adverb 'vorweg' (beforehand/ahead) is made up of 'vor' (before/in front of) and 'weg' (away/way). This combination can be understood both temporally ('before') and spatially ('in front on the way'), which explains the two main meanings.
Vorweg eine kurze Bemerkung: Die Frist wurde verlängert.
First, a brief note: The deadline has been extended.
When 'vorweg' (beforehand) stands at the beginning of a sentence, the verb follows directly after it and then the subject. This word order is called inversion and emphasizes the introductory information.
Vorweg möchte ich mich bei allen für die Hilfe bedanken.
First, I would like to thank everyone for their help.
A very common and useful phrase is 'etwas vorwegnehmen' (to reveal something in advance / to anticipate something). It means to reveal a result or information before it would actually be its turn.
Ich will nicht zu viel vorwegnehmen, aber das Ende des Films ist sehr überraschend.
I don't want to give away too much in advance, but the ending of the film is very surprising.
In the meaning of 'to go ahead', 'vorweg' (ahead) is often used with a verb of movement and a person in the dative. The dative indicates whom you are going ahead of.
Der Guide ging der Gruppe auf dem schmalen Pfad vorweg.
The guide went ahead of the group on the narrow path.
first / for now
'zunächst' and 'vorweg' can both express that something comes before something else. 'zunächst' is often used for the first step in a sequence: something else happens after that. It can also mean that something applies only for the moment. 'vorweg' fits especially well when you say or do something in advance, before the actual topic begins. Learners may confuse the words because both have to do with 'before'. A practical guide: 'zunächst' is more like the first point in a process, while 'vorweg' is more like a piece of information or an action before the actual beginning.

Vorweg möchte ich sagen, dass sich der Termin geändert hat.
To start with, I would like to say that the appointment has changed.
Zunächst prüfen wir die Zahlen, danach schreiben wir den Bericht.
First, we check the figures; after that, we write the report.