Which one you choose is a matter or personal taste. Concentrate on breaths in and out. You can accompany this by counting your breaths using the numbers 0 to 9. You can visualizing images of the numbers changing with each breath. Alternatively you could visualize health and relaxation flowing into your body when you inhale, and stress or pain flowing out when you exhale.
Some people like to focus on sounds.
Whether or not this is practical depends on your lifestyle. This can be a very refreshing and pleasant way of meditating. Here you create a mental image of a pleasant and relaxing place in your mind. Involve all your senses in the imagery. If external thoughts or distractions wander in, try to make them drift out by attaching the thoughts to objects and then move the objects out of your mind.