Luckily, KSP2 includes a thorough tutorial system that I cannot recommend enough.
More importantly, you’ll learn how to play the game!
But for proof of concept and a good rule of thumb, remember the Big Four.

This is where you’re able to view the Engineer’s Report and Trip Planner.
Merged projects can also be used to work on multiple vehicles side-by-side, incidentally.
This is where you determine the stages in which the rocket will deploy on launch.

Helpfully, they’re numbered for your convenience.
If the navball appears solid brown, that’s because your craft is in a nosedive!
Pull up, pull up (using the “W” key!)

to avert disaster and give yourself at least a little more time to work out what’s going on.
This might be what you want!
Or it might not.

Hope you packed a parachute!
It suggests that even the devs struggled playing their own game at times, and that’s OK!
Rocket science is hard, and Kerbal Space Program 2 aims to emulate that reality thoroughly.

What’s more, the point of the game is arguably what you learn from your own experimentation.
Trust me, this advice comes from first-hand experience.

