To be "an empath" means you have the ability to feel another's fears and concerns. In the most direct sense, you have a sensitivity of empathy that seems to extend outside yourself, as if you can sense another's concerns, from their own perspective. This ability is a variation of what some term a "psychic" ability, or the ability to "know" what others are thinking.
Human beings have a spiritual component to their existence that ties them all together in perception, not unlike schooling fish or a flock of birds. This gives them the ability to perceive dangers and move in accord as one unit. Spirit is this "like-mindedness" among social groups. Family members, because they have the same experiences we have, are the ones we most likely feel this empathic connection with, to the extent you may "know" what a sibling is thinking, merely by looking at how they react to a circumstance.
But what of empathic feelings that spring up when around strangers? What you are keying in to is again the similarity of that person to your own experience and your personal understanding of how that other person reacts to that experience.
Empathy is honed through the development of spiritual strength. You can attain that strength through benevolent spirituality, or through manevolent spirituality. The key is to choose a spiritual platform that brings out the best for all involved...so one must be VERY careful not to fall victim to a spiritual doctrine that could lead into dangerous places.
Simply on the basis of your wanting to develop your "skill". you could find yourself among those who only seek to take advantage of you, or could do you harm. Remember, as you can be empathic, others can sense what you are feeling as well. This is the number one reason people fall victim to the manipulation of others, or are "sold" on ideas that really don't provide them with worthwhile benefit.