A key concept is to take time before you begin painting to decide what you want the final work to look like.
The initial lay-in should support this vision by making sure everything you want to portray fits in the frame! (If you don't put the whole head in a portrait, for instance, you risk a scathing rebuke from @Madwurmz who has a pet peeve about such things).

You don't have to draw everything in detail at first, but at least make sure you have a place for it. The simpler, and sooner, you can do this, the easier it is to see whether the layout works from a design perspective, or not. If the initial layout doesn't read well, it may be best to try out something else before spending a lot of time on details.

Compositon can help the viewer identify what the painter thinks is the main focus in the painting. Some ways to draw attention to an area include

a. Value contrast. The eye is drawn to the darkest dark and lightest light in a painting. You can place these next to each other in the area you want people to notice, making other values outside of the area of focus more subdued.
b. Color saturation. If most of your painting has muted tones, and the center of interest has more saturated tones, the eye will be drawn to the area of interest.
c. Level of detail. Some paintings are simple, with little detail. Some are more complex. Both can read well through composition. However, you want to avoid having high levels of detail in every parts of the painting, because then it is hard for the brain to focus on what is important. If you put more detail in the area you want to emphasize the eye is drawn to that. You can make the rest of the areas less detailed, as they are only supporting elements. Vignette or Bokeh effects in paintings are also applications of this concept. d. Variance of shape. If all of the shapes in your painting are curves, except for the main area of focus which are straights, this stands out immediately. The reverse is also true.
e. Directing elements. Support elmeents in the painting can "point" towards the main focus.
A shoreline, row of hedges, counter top, telephone line, etc. can run in a direction that leads the eye to the area the painter wants the viewer to see. Be careful, however, to not make lines that unintentionally lead away, or worse yet, out of the painting.

If you have four figures all looking at a fifth figure this draws the mind of the viewer to see what they are looking at, and to focus on the fifth figure. As social creatures we want to know what has captured the attention of others! Most times you want to avoid figures near the edges looking out of the painting, or moving out of the painting. This can take the attention from the painting itself as our mind wants to find what they are heading towards. However, some artists have used this to emphasize something mysterious, so that everything in the painting is drawing the viewer to one point outside the painting. So just make sure the elements in the painting are producing the intended effect, to draw the eye in, or out of the painting.
When you are nearing the end of your painting evaluate from a distance the whole of the work to see if there are things that are distracting from the main focus. You may have to eliminate or de-emphasize these things. f. Use of compositional schemes. Some artists arrange the elements of their painting using recognized successful patterns. These can highlight the area of interest. These might include the rule of thirds, golden ratio, pyramid, C curve, S Curve, L shape, etc. The elements in the painting are arranged in ways the brain likes to see them.

While you can stumble into these arrangments without realizing it, these are usually the result of planning ahead. You may need to reposition elements to fit the scheme.

A simple compositional concept that appears in many paintings is putting the center of interest at the intersection of large horizontal and vertical elements.
For instance, in the example image for this tips tutorial, the water, hills, boat, etc. are generally horizontal. The verticle sail overlapping these draws the eye.
Ian Roberts, who has a Youtube channel focused on composition, discusses that idea here:

https://www.youtube.com/watch?v=sjqbounftBs

You can see this play out in a photo example, and a painting by John Singer Sargent.
If you want to learn more about composition he has many videos discussing concepts.

To get the composition to read well you often have to decide what to leave in and what to leave out. Or in some cases you may leave an element in but change the shape to move the eye the direction you want.
Cropping is the most basic part of this. If you are painting a landscape from a photo someone has already cropped the scene, taking what was in the environment, and framing it with the camera.
You may further crop it from there, either literally cropping the reference, or just ignoring the part you don't want.
And even if you are painting from imagination the edges of the painting are cropping the scene, so you want to think through those boundaries.
Beyond the edges of the painting, remember, you don't have to paint everything you see in a reference, or in a life painting session.
As an extreme example, if you had a wonderful photo of your friend from a vacation, the best picture of that friend ever, but it was taken with a significant other who broke up with them on the vacation, leaving a terrible memory, you might paint the friend, and leave out the SO, and the vacation scenery!
If you are painting a landscape, not every tree may be necessary. Not every bush may be necessary. Choose which ones help define what is important in the scene for you.

Related to detail control, Don't try to draw everything. Sometimes inferring things is good, and helps the viewer to engage with the painting.
This saves time, saves the most detail for the focus, and helps simplify the scene to help the brain of the viewer understand it.
In this video Marco Bucci gives examples from historic paintings of simplification, discussing merging of shapes.

He then does a demonstration that further shows these concepts, along with discussing the center of focus, lost edges, etc. and how to use them to create mood.
https://www.youtube.com/watch?v=Nap7dwHjD9Y

He has great, short videos that discuss art concepts.

https://www.youtube.com/watch?v=-ZknWKTpc90&t=501s

This video discusses how to design the shapes within your painting/drawing to appeal on a graphic level. It explains how to intentionally make clean, easy to understand shapes, to improve readability.

COOL!

A strong composition is like the solid foundations of anything. If it ain't right, the whole structure will crumble.

ah yeah! I use the golden ratio alot, that is almost the rule of thirds , but differs from 66% to 61,8%. The golden ratio has a pleasing balance that comes from a 'perfect' formula , the size of some measurement along some other measurement can be placed in a balance that meets the mathamatic ratio simplified it is a + b = a / b = 1,61803.
Personally I think approaching the canvas, the start of a drawing, the composition, is some very juicy moment, and even when I have some ok result I like to start over a few times , so the construction of the first lines are thought through.

BRUH WHAT. this looks like a photo :0

Bruh ya'll so good at art but im only good irl like what?!

@quinn_t it takes a while to get accustomed to doing art on a screen. But don't give up. Most folks who enjoy traditional art can get the hang of the tools with a little practice.