Shorts is a vertical format, right? So the main thing is make sure you're shooting in portrait orientation (9:16), not landscape. On iPhone this happens automatically if you hold the phone vertically, but check your camera settings that the resolution is set to maximum - 4K is preferred. As for lighting, in natural daylight indoors try to position yourself so the window is in front of or to the side of you, but not behind (otherwise you'll be backlit). If the light is uneven, move a lamp or use a white sheet of paper to reflect light onto your face or subject. The key thing is don't shoot in dim light, even if it looks okay on your phone screen; it always looks better on the phone than it actually does.
About framing: in Shorts viewers see the video fullscreen, so leave the edges empty or just don't put anything important in the corners. Keep your face or main subject in the center or slightly above center. Don't move the camera abruptly - even when shooting on an iPhone, smooth movements look much better. If your hands are shaky, try bracing your elbow or using a simple stand.
When editing in YouTube itself or in CapCut, adjust the contrast and saturation - often video from an iPhone looks a bit washed out, and a little color correction makes it pop. But don't go overboard with effects at first, better to focus on clear audio and making the video interesting content-wise.