Firstly, the most important stat is raw production. Population is your primary input for this, so building farms and cities, while not necessary early game, should be a consideration when building out your planets. The factors that constrain your ability to make big populations are morale and food.
Early game, you will want some techs from the governance tree that give you access to some low level finance buildings. This can help keep you out of deficit as you expand. Depending on how fortunate you are with discovering colonisable planets, you may want to designate roughly a third of those you find and colonise as specialising in finance. While you will mix up your planets to some extent, you still want to have an overall plan in what each of them will excel in.
Starbases are also another powerful tool for your economy, whether its capping strategic resources for specialist buildings, setting them up to boost your planets. There are finance modules that can help you!
Also, trade is a really important part of this game. Don't be afraid to set up trade routes for extra cash. The further away, the better. And also, don't hesitate to trade resources and techs for more of the same in the diplomacy screen. Treaties can be exchanged for money (a lot of money), or just straight up trading techs if you're short on cash. Trading is absolutely fundamental to this game.
Also, don't let the learning curve put you off. There's a lot of fun to be had just by learning the game
And, I highly recommend the Crusade expansion. It's great, and adds a lot to the game!