Learning to fly straight and level will form the basis on which the rest of your training will build.
Straight and Level Flight
As per the definition, straight and level flight means to maintain an altitude, maintain a direction and to keep everything in balance. This is the critical first step in controlling the aircraft, as any manoeuvre will start from straight and level.
Your instructor will start by getting the aircraft in straight and level, so you can see what that looks like! Remember, as PPL pilots, we fly by VISUAL reference to the horison OUTSIDE of the cockpit. So take some time to focus on where the horison "touches" the aircraft, for example, 4 finger widths above the glareshield on either side of the windscreen.
This will differ for each person as the position of your seat and your physical length/height will be different to others.
Once comfortable, your instructor will show you what straight and level looks like in different aircraft configurations ie. During slow slight and with flaps.
In the video linked below, Carel takes you along on his Straight and Level Lesson.