Being able to clearly visualize what one wants ones magic to accomplish is absolutely central to effectively doing spellwork, ritual work, and any other form of magic. After all, if you're going to cast a spell you want to be very clear on what you desire as an outcome in order for the magic to have a path to travel. Everyone can visualize, we do it all the time when day-dreaming. But being able to focus and hold a focus on a specific image is more of a learned skill that takes time and practice to develop. The following is an exercise that can be used to develop your ability to visualize a specific object/outcome clearly and to hold that focus at will.
Take an object and study it carefully. For the sake of this exercise lets take an apple. Hold it in your hand and feel the texture of the skin. Smell the scent of the apple. Look at the color. Are there any bruises? Holes in the skin? Discolorations? Keep studying the apple until you think you have a good grasp on what an apple looks, smells, and feels like.
Now, close your eyes and try to see that same apple in your minds eye. Don't worry if the image isn't really clear yet or if you cant hold it for too long. If you start to lose the image, open your eyes and study the apple again.
Don't strain yourself in doing this. Practice for five minutes or so seeing if you can hold the image a little longer and have it get a little clearer each time.
Continue to do this exercise repeatedly over the next weeks or months. As you become better able to hold the image try adding some changes to it. Now you not only see the apple in your mind...but perhaps you can also smell it or feel its skin.
Try rotating the image of the apple in your mind. So this time you're looking at the stem and perhaps next time you're looking at the bottom of the apple.
As you get more comfortable with visualization you can get more creative in the sorts of changes you want to make in the image of the apple. Try changing the color of the apple from green to red. See if you can make the ripe apple shrivel and dry up. Even try seeing the apple through the full course from bud to blossom to fruit to windfall.
I'm using an apple here as a teaching tool because its easy to work with. But of course you don't have to limit yourself to an apple. Once you begin to gain skill with visualization you can work with any image you want.
Its important not to rush things or to get frustrated if you're having problems. As I said before, visualization is a skill that takes time to learn. Each of us learns at our own pace. Like any other exercise it takes time to build up those mental muscles to do what we want and there are no shortcuts. Enjoy the process and keep at it on a regular basis and you'll be surprised how well you will be able to hold a visualization in your mind.