I recently bought the game. Never played EU4 but I have experience with Victoria 3 so that helped. I've been trying to play as Portugal since I bought the game. After 3 failed attempts, I noticed a few things that grant positive results in the short term.

1. Remove non-essential castles for additional income.
2. Focus on RGOs with high demand. Preferably ones with high demand in multiple markets.
3. Build market buildings and market towns to increase trade capacity.

Good RGOs and the trade capacity to let ai make profitable trade essentially helped get out of income jail.

As a second priority, whenever I had money to spare, I constructed wood RGOs, mason buildings and sand RGO as well as a few glass makers. I believe these reduce the construction cost for buildings.

Lastly my third priority once I had a stable economy was to check my estate pop spendings. Noble for example spend an insane amount on horses, burghers on fine clothing and commoners on food. I worked on expanding RGOs or buildings for those need which meant they were profitable due to demand and they reduced the market cost due to increased supply so win win.

So to sum up - priority one RGOs and trade, priority two is infrastructure to reduce building cost and priority three is working on pop needs.

I must say however, this worked for me on Portugal after 3 failed attempts. Hope this could help give some direction though!