The economy whistles oh boy.
Ok I’ll be brief… well probably not, but I’ll try.
First off tax base. Each province has a tax base (you can see it in the top left corner of the province detail screen) which roughly determines your income. It’s not 1:1, but all you need to know is higher tax base = greater income. 1 tax base is roughly the equivalent of 1 gold per year, but it’s only a rough value.
So to build economy build up provinces. The church adds +1 tax base in a province, and is something worth building anywhere. Other buildings can increase tax by a %, those are generally best built in high tax base provinces. +20% of 8 base tax is better than +20% in 3 base tax, for obvious reasons. In general build starting with your most valuable provinces first, and expand out as monarch points and gold allow.
Prioritize gaining high base tax provinces. If in a war you can take a province that is also a trade hub, do that. They’re very valuable.
Buildings and base tax are longer term investments. They can be massively valuable over time, but the effects of any one are relatively small in the short run. A 1 base tax province isn’t ever going to provide much economic value, so buildings are mostly a waste there.
If you colonize focus on trying to get a colonial nation in a region (5+ colonies) rather than scattered colonies. 5 provinces forming one colonial nation will likely be more profitable for you than 15 scattered so that they are all directly controlled. It might seem counter intuitive, but the overseas penalty is SEVERE. You will almost always get more money from colonial nation tariffs than if they were directly taxed because of this penalty.
Finally trade. Countries can exert trade power at trade nodes. This is determined through provinces, several buildings increase trade power at the local node, and light ships. So find the most profitable trade nodes nearby and try and either collect trade, or forward trade, from there. If a trade node is in a province you control, really focus on trying to build up trade strength there. More buildings and such. Trade propagates power to connected nodes, but it’s complex and a post on it’s own. In general collect from your home node (done automatically) and have your merchants forward trade from the neighboring nodes, forming a chain. Forwarded trade increases it’s value, so play around with your merchants to observe the effects. Send a merchant next our to forward trade, and see how that impact your trade income at home. It’s pretty low risk to experiment, so please do until you get a grip with it.
Do not forget light ships. A fleet of light ships protecting trade at a node you are collecting from, or forwarding from, has a huge impact. In fact it’s not unusual for me to have >50% of my trade power in a node be ships. Put an admiral with the fleet if you can, he helps increase the fleets power.
Trade income is based off of what percentage of total trade power in a node is yours. If you have 10 power, but there is only 20 total, you get half. In a valuable node like Antwerp you might have 200 trade power, yet only get 1/3 of the trade value. Pay attention to who has how much power. If you have 13 power, and England has 240, your investment is going to be extremely high to get a pittance from a node. You are probably better off collecting elsewhere. Again play around with this. Just remember fleets only contribute to trade I you set them to protect trade at that node. Sitting in port at that node does nothing.
Just note that this is all pre Wealth of Nations (been busy since that dropped) so this might be out of date. That said it still should be roughly accurate enough to get you started, especially if you don’t have the DLC.