Good advice above, thought I would I would add a few things I have noticed (as a newb). You need to be really careful at the start of the game not to blow your entire wad in the first round on units. The upkeep will kill you until you build up your infrastructure.

Make sure you don't have too many castles. The Iberian penninsula has 4? in total. You really only need one in any isolated geographical area. In unisolated areas, go for a ratio of 1 to 4 (or 5 if your nation has good city units).

Build your population up with farms. More people = more taxes, bigger cities and therefore better trade upgrades. Squalor can be managed with the various inns and townhalls.

Send diplomats/proncesses everywhere to obtain trade rights. Once you build ports your revenue will jump markedly.

One of the first things you build in every city is port. In the Iberian pennisula every province has a port (except Cordoba maybe?). This should be either preceded or followed shortly by roads.

Get your merchants experienced close to home by sitting on a decent tradeable item for 5-10 turns. Then put them on a boat and send them to Antioch, north africa or some of the other areas mentioined above. It is important to upgrade them forst so they are not easy takeover targets.