Prussia would make for a great second game, in my opinion. First, you may want to try to stabilize the economy of some quiet country, just to get a handle on some of the basics. Promoting Clerics for the Literacy gain, Militancy reduction, and Research points would be the usual recommendation for your National Focus point(s), but you may need to promote Bureaucrats in a few regions first, to improve your income. Don't over-do it, because paying for more bureaucrats than you actually need is a waste of money. Once you've got a stable income, promoting Clerics up to AT LEAST 2% of your population is recommended (the max which will produce Research Points), and can be helpful up to as high as 4% (the max which will increase Literacy). Once serious industrialization takes hold, you might want to promote Clerks instead of Clerics, for better industrial productivity.
Taxing at 100% isn't really 100%, since your tax efficiency is probably terrible, so only a small portion of that will actually be collected. Feel free to boost taxes to the max at the start (some government forms will limit how high you can raise them). Hovering over the various sliders should give some insight into what's ACTUALLLY happening, although it's far from perfect in several cases. You can begin lowering tax rates as your tax efficiency rises and you start to gather a decent surplus in the treasury. Ideally, you can set general tax rates for social "classes" and individual salary sliders for soldiers, bureaucrats, and clerics to balance them, so that they get their basic and life needs, but don't get TOO many luxuries and promote out to some other profession. Achieving a balance, or an intentional imbalance, will shift population to and from various "pops" over a period of years. The REAL trick to this game is figuring out how to manipulate your pops through indirect actions, but don't expect instant or even reasonably rapid results.
The problem with playing Prussia right away is that you need to learn about the micromanagement-intensive Spheres of Influence mini-game right away, or else Austria will run rampant and take away a lot of your sphere members before you figure out how to stop them. Other than that, they're pretty much "easy mode". You can form the North German Federation without any wars, but will need to take Alsace-Lorraine from France by force to create Germany. Playing France or Austria, you can prevent the formation of Germany by holding even a single North German state in your sphere, so NGF never forms.