That we have the capacity to ask this question is a testament to our progress as a species. Each person will answer this question differently, even though there might be some overlap in ideologies in quite a few places. This is a big question. For some its hard to imagine why anyone at the common individual level would matter at all.
This isn't the only question one really needs to answer in life. But to understand that you have the potential to have a real and significant effect on the world around you is of vital importance to your well being. Knowing that you matter will have a profound impact on the way that you conduct yourself in life. Wouldnt it encourage you to maximize what you achieved in life?
Most of us dont seem to think about this question at all. At the very least they have already assigned themselves some deserved degree of importance and without further adieu moved on to the equally important question of 'What do I want to get out of life'. This too is an important question. It will change depending on your interests and your political/geographical/economic conditions. Answering the question of 'why do you matter' is never a prerequisite to answering this question.
So, Why do you matter?
If you have ever engaged with any other human being in the workplace, at school, at parties, in church, while volunteering, you will have experiences of where you directly effected another person. You will most certainly have experiences of other people effecting you. Your parents or carers being an obvious source of these experiences. You will know how others can harm or improve your enjoyment and experiences of life. This understanding is very important in considering the source of morality.
This knowledge that others can harm or improve your state of life should lead you to the understanding that you can do the same to others. This is a lesson that every toddler learns very early. Sometimes it is good to remember these simple lessons as they can bring clarity to how you perceive the world through the complex eyes of an adult. Remembering this certainly helps me out and it is the basis of my entire view on life.
You matter, and I matter, based on the effects that we can have in the world. It might be the case a person with greater resources can have a greater effect. But does any of this matter if they do not have the capacity for personal control, choice and responsibility to begin with? What is more important than the capacity for control at the minimum level? You matter because as a human being, you are unique in your capacity to intelligently interact in the world.
You can have children, attend school and university, be nice to the people you are serving at the checkout, get involved in your local community. Have a meaningful relationship with a significant other. There are limitless possibilities. Just do your best to do what is best.