Rule 1: There are no rules.
There are a few guidelines though. Things about visual compositions that make the image more beautiful or emotionally stimulating.
Rule 2: Even though there are no rules, this rule still applies almost all the time. Horizons must be level. The most sickening photos are those where the you have to tilt your head to appreciate the subject. Try to get your horizons level in camera every time. You can tweak a degree or two in Lightroom afterwards if necessary.
Non-Rule 3: The Rule of Thirds. When the subject is dead centre of the frame, it is invariably a dead boring photo. Move your subject off to the side.
While looking through the viewfinder, imagine the frame has two vertical and two horizontal lines, splitting the frame into nine blocks. The guidelines suggest that putting your subject on one of the lines is a good thing. If you can get your subject's eyes onto an intersection, even better.
This goes for horizons too. It looks more pleasing when the horizon is on one of the "third" lines.
Non-Rule 4: Background distractions like trees and lamp posts sticking out of your subject's head isn't a good thing. Likewise fire hydrants, signposts, unnecessary people and anything else that don't add to the photo should be excluded. Try move yourself or your subject just a step to the left. Or jump to the right.
Non-Rule 5: Space.
Non-Rule 5a: Give your subject space to move. If your subject is walking from left to right, place them on the left side of the frame so they have somewhere to go.
Non-Rule 5b: Leave some blank space (a.k.a. negative space) like the sky or a wall in your frame to create balance and harmony.
Non-Rule 6: Get closer to your subject. This may seem contra to Non-Rule 5, but work with it. And Non-Rule 3.
Non-Rule 7: Be odd. An odd number of elements in a scene help to make it more dynamic and interesting.
Take a photo that applies at least three, if not all of the Rules/Non-Rules suggested here.