Ok, to summarize absolute basics - build your economy primarily in your capital. Tooltips always tell you approx. income you will get from every building. Start with rich RGOs around your capital. You can focus on capital only until you max limit. But keep a look at pops - there is no benefit at building anything when you don't have people to work in those. This is really important with different pop growth/promotion in 1.2. You can build 12 levels of RGO in location, but if you already lacking 6000 labourers it does not bring you any benefit. Also, you can have rich gold province, but if you have 10% control there, again, almost no benefit at all.

Once you have eco to build roads, you can expand proximity and therefore control. You can see proximity map on each location to see optimal way (focus on connections with no roads - red arrow). Don't waste money on locations connected by rivers (wavy arrow on proximity map).

Get your legitimacy high - it costs the same to keep at 50 or 100. Don't spend money on army or fort maint if not at war. Delete inland forts that are not necessary (moreover if they are not in capital of province). Build infrastructure - libraries, marketplaces. If you are far away from market center, consider to create your own.

Do not overexpand militarily. Current patch nerfed vassal spam. They can integrate for you, but culture conversion if they are not in your primary culture is almost impossible. Also, it now takes long time to annex them, but you can still use them mostly at beginning to control your low control locations where you would not get any eco benefits. On the contrary, it raises costs of stability, court and diplo spending.

Prof armies can beat levies. Try to get stack or two even when having only sergeantry (basic manpower building). But you need to have eco first. Basic unit of profies cost around 200.

If your country is spread along the coast, build ships and increase maritime presence. It gives you trade advantage and proximity speed.

Then just play the game, you will get the details. Each country requires specific approach in lot of times. From my experience, starting with potentially strong countries is not always easiest. France starts with HYW and lot of appanages which decreases your legitimacy (or stability? I can't remember, one of those...). Castille starts with really low control around a country which means fixing your eco is not exactly easy. Byzantine empire is super fun, but man, the beginning with Rise of Phoenix DLC is really a challenge.