Don't center your subject. Imagine four lines —two vertical, two horizontal— in your camera's viewfinder which divide the picture into thirds vertically and horizontally. Try to place your subject on one of the points of intersection of these lines. Then think about what is in the rest of the space. Is it empty or crowded/distracting from your subject?
Be aware of deadspace and objects which clutter your subject.
Don't just immediately take a picture, but think about its composition in the frame of your image and what else is in there. Are their spacial relationships pleasing or could they be improved by moving around? By changing the orientation of the frame?
Be aware of the edges of your frame. Are you cutting something off? Is that acceptable to you or not?
Be aware of the direction of light —look for shadows— and how that falls on your subject and scene. Is the subject front lit, back lit, side lit, top lit, soft lit? All these types of lighting have different effects.
Think about whether it is worth coming back later to take a picture from the same location under different lighting conditions, assuming you are able to. Remember Golden and Blue hours.
Be aware of shadows in your pictures and in your environment more generally even when without a camera. Much fun can be had depending on the lighting conditions and playing around with this.
Don't be afraid of the flash. Learn what you can do with it.
Are you exposing for middle grey (what your meter tells you to expose), highlights (which will put darker elements into profiled shadows) or shadows (which will blow out highlights of detail)? Again, different effects with different looks.
Don't be afraid to "waste film" by bracketing; take multiple pictures of the same thing with different exposure levels or other effects.
Have fun playing around with different focus, depth of focus and exposure levels to get different effects.
Don't be afraid of slow shutter speeds and motion blur. Just do it intentionally. Learn what it can and cannot do. A neat trick is any tripod exposure of 5 seconds or more in a street scene will cause most pedestrians and vehicles to disappear. Learn the value/use of a neutral density filter. If you shoot B&W film then filters are your friend.
Learn about fine art rules of composition: Rule of Thirds, Rule of Odds, Rule of Shapes, Rule of Lines, Rule of Rhythm, Rule of Space (deadspace and sub-framing).
https://www.youtube.com/playlist?list=PLGEE7pGLuppTEjrviNCTwDHA00VsMzsAl
Study the work of acclaimed photographers or anyone who's work you like who have come before you. If you are not already subscribe to Ted Forbes' most excellent YouTube channel The Art of Photography where he explores the work of many "past masters." The whole channel is great though. Get large photography books from amazon or your local library or the work of better photographers than you on Pinterest.
Ted Forbes has an extensive YouTube channel worth of quality content:
https://www.youtube.com/c/theartofphotography/playlists
Try to reproduce, or imagine what it would take to reproduce, older acclaimed works yourself. It will teach you how they did it and you will have that in your back pocket for the future.
If you are unsure of a picture DON'T take it. NOT taking a picture forces you to think about what you really want and makes you more disciplined. Yes you will loose many good ones you could have gotten, but what you end up with tends to be of higher overall quality.
When you don't have a camera study your surroundings in your daily life for how you would photograph it if you could. Study the lighting, the composition, pre-visualise your framing. What better position could you move to to get a better shot? Always be thinking about these things, not just when you have a camera handy.
Learn to "pre-visualize" like old-timer film photographers had to do what a picture would look like before you bring a camera up. Even in these instant-gratification times of digital photography. This will help you "see" photographically and improve your ability when you do bring a camera up to your eye. Even if you can "fix it in post" much more easily these days than back in the darkroom days, you will be a better photographer if you don't have to and your picture is good, or nearly good, right out of the camera.
Don't be a generalist photographer. Figure out what subject or few subjects you really like and mostly only photograph that. Apply the 10,000-hour rule and get really good at photographing X type of subject. Make yourself the best at some photographic niche and don't venture away from it, at least not until you're done with your current subject and willing to near totally devote yourself to the new one. If you don't specialize you'll become mediocre at everything.
Finally; Remember that photography is an art and is about fun and aesthetics. If it
LOOKS right it
IS right, regardless of anything else I or anyone else says. It is about whatever you subjectively feel is interesting, memorable and pleasing.