One of the most basic rules in visual art composition is called The Rule of Thirds. It's been used throughout history to help determine placement of elements within a painting. Understanding how this rule works will help you lay out your composition and determine your focal point in order to achieve the effect you're after.
Simply put, the rule of thirds states that if you divide your composition into three equal parts horizontally and vertically, and then place the elements of the painting along these lines or at the point where the lines intersect, you'll achieve a pleasing arrangement. Let's take a look at an example.
In the painting above, you can see the pitcher and the cup have been placed perfectly along the lines. The painting in the background has also been placed at the intersection of two lines which brings it into focus. Notice how you view this painting. What's the first thing you see? How does it make you feel?
Now let's look at the same painting with the elements arranged slightly different.
In this example, the elements of the painting are placed between the lines. What happens to the focal point? What's the first thing you notice? Do you feel anything different when you look at this arrangement?
What's great about the rule of thirds is that it's not really a rule that has to be followed exactly. It's a guideline - but an important one to know and understand. You've heard the old saying "you have to know the rules before you can break them?" That certainly applies with the rule of thirds.
Understanding how the rule of thirds works is very important as you're developing your painting or drawing skills. Choosing to follow the rule exactly or choosing to bend the rule is a choice you'll make depending on what you want to emphasize and draw attention to.