Learn Russian: Сначала мы поедем в город, затем на море. - First we will go to the city, then to the sea.

Сначала is used for a chronological sequence of events, meaning "first in time" or "initially". You would use во-первых when listing arguments or points in a discussion, which would usually be followed by во-вторых ("secondly").

Why do we use поедем here instead of пойдём?

Both words mean "we will go", but Russian verbs of motion specify how you travel. Пойдём is used for trips made on foot. Since going to a city and then to the sea almost certainly requires transportation (like a car or train), we use поедем, which is the perfective future tense for going by vehicle.

Both the city and the sea are our destinations, so why use в for one and на for the other?

Both город and море are in the accusative case here because they are destinations of motion. Russian uses в for defined boundaries or enclosed spaces, like a city (в город). It uses на for open areas, surfaces, and certain geographical features. "To the sea" is a set phrase that always takes на (на море).

Could I say потом на море instead of затем на море?