You can use a joystick, mouse or keys, or a combination of them, i'll use the Default keys for this tutorial and although I'm using a Stuka the same takeoff procedure applies to all the planes in IF.
There's the Stuka, and notice that i (as Player) am a schreck guy to show that you can be ANY unit, you don't have to be a pilot
The Stuka is boarded from the left-hand side, click mousewheel when the steering wheel icon appears-
The view will be external 3rd person like this-
Hit *(num) to get the freelook view and slide the mouse around to get a good idea of situational awareness. The runway stretches away towards a treeline so we'll have to do a 90-degree turn to line up for takeoff-
Now the (gulp) tricky part..
Hit *(num) again to toggle off freelook and hit 'Q' to start the engine. Hold down Q until the plane starts moving, then release it and stroke the mouse right several times to rudder the plane around til you're lined up with the runway.
Go to freelook again *(num) if you like, to check you're positioned ok-
You can stay in external 3rd person view or hit Enter(num) to get in the cockpit like this-
Hold down Q to increase engine power and keep holding it down. You can apply a touch of rudder (mouse stroke) to keep straight as you hurtle down the runway, and when your speed gets above 100 (arrowed) apply a touch of up elevator with the mouse, or with the down arrow key and you should comfortably clear the trees at the end of the runway like this. I use freelook *(num) as here because I don't like being cooped up in the cockpit for takeoff. DON'T CLIMB TOO STEEPLY or you'll slow down and stall.
Finally release the Q key when your speed has built up to at least 150 kts, or hold it down longer if you want to go faster-