When trying to make sense of what the study of chemistry entails, it helps to distinguish chemistry from other sciences like physics. In physics we study phenomena like the rotation of the moon around the Earth or the falling of an apple to the Earth. In physics we are not directly concerned with what the moon, Earth, or apple are made of. The law of gravity that accounts for the rotation of the moon and falling of the apple applies equally well if the moon or apple were made of different "stuff", and makes the same predictions of motion for objects of any material as long as the masses and distances from each other are the same.
Chemistry on the other hand is concerned with the "stuff". It asks how the particular kind of "stuff" that makes up the Earth, moon, or apple came to be, and how the "stuff" it is made of can change in the future. It is the systematic study of how one kind of thing ("stuff" or substance) can be turned into another kind of thing – a different substance – different "stuff". This kind of change is called a chemical change and can be distinguished from physical change which does not involve one kind of thing turning into another kind of thing.
Chemists distinguish two kinds of substances or kinds of things – elements, and compounds. The simplest substances are called elements. Everything in the physical world - rocks, trees, animals, stars, and planets, the air we breathe, and the ground we stand, are all made of same 100 or so basic elements. Although everything is made of the same elements, it is very uncommon to find samples of pure elements in nature. In nature we mostly encounter more complex substances called compounds. This was a source of confusion for early scientists, and is possibly confusing for you as well. It is seems natural to think that substances common to many things, like water, are elements, but as you know, water is not an element, but a compound of the elements hydrogen and oxygen. We will try to clear this up in section 1.2. For now we can say that chemistry, unlike physics, concerns itself with the different kinds of things found in nature, and how to turn one kind of thing into a different kind of thing. For example, although neither sand as found at the beach nor solar panels are pure substances, we can capture the spirit of our definition by saying that chemistry as a science is interested in improving human life by turning material like sand, into a far more useful thing, like a solar panel.
To get started with this way of describing the natural world, you should start by learning the names of the most common elements. A list, and opportunity to learn their names, is given below.