A mantra is simply a word or phrase that you repeat over and over to yourself. This is usually done silently, but can also be used like a chant. A mantra can be a meaningless sound, a religious word or phrase, or something with personal meaning to you but always keep it positive. Mantras have been shown to have very positive physical as well as mental and emotional benefits. You can meditate on a problem or idea, a thought or feeling, or a piece of information.
Many people simply use their breath as a focal point.
However, regular meditation is needed to gain the full benefits of meditation. With practice your level of relaxation will deepen. Your attention span will increase. You'll become more skilled at living in the present moment. Many of the mental and spiritual aspects of meditating will become apparent over time.