You probably installed the game, and you don't know what to do. Here is a Simpleplanes guide.
Nothing about it, just nice plane that you should fly first
This is basically F-18
To make new aircraft, press "New aircraft" button. Then click on "+", then on structucal
Drag "Block", and attach it, like on the image
Then find "Nose cone" part and attach it, like on the image
Then find "Angled block" part and rotate it, using keys "A" or "D" then attach it, like on the image
Then, attach one more block to the aircraft
Then, attach "Blasto J15" engine, that you find in "Propulsion", for your first plane you need weakest engine
The engine needs air, so we need an inlet, "Slim Inlet" - is what you need
Add wing, horizontal stabilizer and vertical stabilizer
Edit wing shape like on the image, by double-tapping wing
Add control surface on the wing
Mirror aircraft
Add landing gear, from "Landing gear" tab
Save aircraft