i tend to build 1-2 scouts rather early to (hopefully) find a few good spots for additional settlements. i play dense monsters, so i usually have to clear some lairs before i can settle another city or 2, but on normal settings, you should be able to find a few spots that aren't heavily guarded.
i like to grow my capital to level 2 before i build pioneers (i start with conclave most of the time if my starting spot has at least 2 essences, but town or fortress starts can work out fine, too - just personal preference). once the cap is level 2, the next goal should be to get at least 2-3 more cities soon. aim for at least one conclave, one town and one fortress in total - preferably with appropriate terrain - conclaves are a lot better if they have essence slots, towns benefit greatly from an adjacent river, fortresses should have either lots of material and an adjacent forest, or several essences [for troop enhancing city enchantments].
fortresses are a lot better at training units, so if you don't really need units RIGHT NOW, build them in your fortress later. conclaves and towns have several great economic/research buildings that will occupy their build queue for most of the game, but fortresses only get the very basic money/research buildings. get the production boosting stuff and the unit improving buildings (forge/training yard and their higher tech upgrades) ASAP and afterwards your fortress can happily spam as many units as you want/need.
the important idea is that you should expand quickly early on, so you have several cities that contribute research and money and a dedicated specialized fortress for decent troops.
it gets more tricky if you can't just go there and plop down new cities. if you actually need units to clear the area before you can safely settle, don't delay the units too long. getting better at the tactical combat system helps a lot in this situation. if you can clear those lairs with 2 spearman units and your heroes, you can settle the spots soon. if you need better weapons and more units before you can tackle them, your expansion is delayed, which can throw you back immensely.
the sooner you get some extra cities up and running, the better off you will be. early expansion is the key to a successful game.