Have you ever taken a photo that you really love but something made you feel like it was missing something? That something may be a slight crop and that small adjustment just might turn your photo into your all time favorite.
There are different composition methods but I normally use 'the rule of thirds' either in camera or in post production. It's super simple. Imagine you have two horizontal and two vertical lines spaced perfectly even on your image. Now if you were to place your subject, the horizon line, etc onto one of those lines then you've just composed using 'the rule of thirds.'
If you're unsure of what to line up then ask yourself what you want to draw attention to. For example, if you're up close then you should want to draw attention to the eye or what tells the story. Unless you have another reason not to :)
It can be great to place things in the middle while still using the rule of thirds also. And it is totally okay to be off a bit, as long as the image has improved to your standards.
If your subject is facing a certain direction, try to face them in the area with the most space, as if they could walk straight in that direction if they chose to.
I personally like to show more sky or above the subject but others may like the foreground more. It all depends on your goal for what you want to say in your image.
Don't forget about looking for or creating opportunities for 'negative space,' it can be quite impactful. Negative space is when your subject surrounded by nothingness or by things that doesn't distract from the subject or clutter your image.