In RTW I, much of the decision is faction based. For example, as Germania the initial concern is fending off Brittania and Gaul and this means ground troops. Of your five starting settlements, only Damme and Mogantiacum have enough population to recruit your best starting infantry unit, Spear Warband, on a regular basis. Germania has naval needs only to the extent of eventually cutting the flow of troops from Brittania itself, so it follows that farming is the primary early need.

An opposite example might be Carthage. Early needs are fending off the Romans but Iberian Infantry and Libyan Spears just don't cut it against Hastati, and Round Shield Cavalry is too weak to engage much in melee with Roman infantry. The best early solution is a powerful navy that doesn't allow the Romans to ferry troops wherever they wish. So ports, and the ability to produce triremes before the Romans is crucial.

In the later stages of a game, port trade is far and away the biggest source of income. In fact, at dockyard level, port trade exceeds all other forms of income...combined. A general rule of thumb for initial development is roads>land clearance>port. These can be upgraded (with faction restrictions) dependent on the situation and whether you need more navy or more ground troops.