You need a plane with wings, flaps, and propellers/thrusters. You can actually turn just about anything into a plane if you do it right!
You can find the plane I used by clicking here

You'll need a good camera angle to see what is in your way! Mainly right behind the plane is good!

Operation Mode, enabled by the ALT key, is crutial when flying a plane. The moment you enable it, the motor automatically starts! You can't move the camera as well. Ready for you to be in control?

Now it is time for you to fly! To fly up, move your mouse down, and to fly down, move your mouse up. A good way of turning is to turn the way you want to go by moving your mouse left or right, then move your mouse down.

Want to make your plane look more realistic while flying? Then lift the landing gear up by holding down PageDown while in Operation Mode!

Tip #1: The flaps are one of the (if not, the) most important part of a plane! This part helps it to fly!

Tip #2: To move the sideways flaps, do Steering while in Operation Mode.

Now you know how to fly, you can now soar the skies like a bird!