Tutorial: Your First Plane
You've just unlocked "Aviation" tech, you have a bunch of contracts that require you to stay at low altitude, and you want to build your first plane? Then this tutorial is for you. (Yes, you personally, you lucky thing! Your very own tutorial.)
First thing you're going to want to do in the SPH is turn on your centre of mass indicator (this is the point that the plane will rotate around when rolling, pitching, or yawing) and your centre of lift indicator (the Aerodynamic Overlay). These should be in the bottom left next to the display of the cost of the aircraft. Enable mirror symmetry to save yourself some alignment effort. Now I'll walk you through a basic aircraft; fancy stuff like science equipment can be added later. This plane will be able to take off, travel somewhere, perform a crew report, and then land.
Firstly you're going to want to make a short fuselage. A Mk1 Cockpit, two Mk 1 Liquid Fuel Tanks, and then cap the back with a round nose cone (use the A/D keys to rotate it as necessary). Now for wings, the "Wing Connector Type B" is the largest you have so far; connect a set of those where the centre of mass is. "Type E" is the smallest you should have, and you want a set of those connected at the back of the fuselage. Then at the top, we'll put one tail fin, centred on the end of the fuselage.
You should have something called an "Elevon 1"; this will be the moving part for your wings. You want an elevon on each set of wings. Now right click each elevon and the tail fin and set what movement each controls; the ones on the large wings control roll only, the ones on the back wings control pitch only, and the tail fin controls yaw only.
Making it move
Now for the engines. You want to start by attaching a Mk 0 Liquid Fuel Tank under the wings, making sure you're mirrored so it goes under both wings. Then you want to put something called a "Small Circular Intake" on the front of the tank, and a "J-20 Juno" engine on the back of the tank. Next you need landing gear. You can either go with four "LY-O1 Fixed" or a tricycle of two LY-01 near the back and one "LY-05 Steerable" at the front; either is fine for now.
Finally you need to make sure it's all balanced, this means the centre of lift marker needs to be very slightly behind the centre of mass marker. If you placed the main wings at the centre of mass and then added the smaller wings behind, this should be correct already. Otherwise, you can either shift your wings till it's right (though this may crowd them near the back), or you can very slightly rotate the big wings so they're slightly higher near the front of the plane.
Hopefully this gets you your first aircraft that can take off and land, which is the biggest hurdle to being able to make KSP aircraft.
Just like with rockets, get some courageous Kerbal in the cockpit and let's get started! Throttle up to full, activate SAS, stage to start the engine (you'll only have one stage here), and start rolling (or sliding) down the runway! As you reach 100m/s, hold S to pull the stick back, and you should be in the air!
For your first flight, it may be easiest to ignore yaw altogether and just maneuver by rolling slightly and pitching. You should be able to navigate fairly readily, and with the superb efficiency of jet engines, you should have plenty of fuel to go anywhere you need to go.
It's said that takeoffs are optional but landings are mandatory. It is also said that a good landing is one you can walk away from. Unfortunately, with only these basic parts, landing on the runway won't be easy; a viable alternative is to throttle down to nothing, then belly-flop in the ocean. Aim to get your speed below 50m/s, and have plenty of water ahead of you; keep the plane level, and pitch up to shed velocity.
As with everything in KSP, experiment, experiment, experiment. And above all: have fun!
This tutorial was created primarily based on a Reddit post by the incredibly helpful u/AnArgonianSpellsword. Thank you and happy landings.