For instance, in air the two main molecules that occur are the nitrogen molecule, which is made up of two nitrogen atoms stuck together, and the oxygen molecule, which is two oxygen atoms stuck together. Scientists write this as N2 for short. And O2 for oxygen molecules.
Molecules occur not just in gases like air; liquids like water and solids like salt also are made up of molecules. Water is made up of molecules of three atoms: one oxygen atom and two hydrogen atoms. You can write H-O-H (or H2O for short).
Not all combinations of atoms are possible: the atoms like certain shapes and combinations but not others. You cannot easily have a molecule H-O, for instance. Or H-O-O. But you can have H-O-O-H (H2O2). This last one is also called hydrogen peroxide. You can use it to bleach(to cause to become white or pale) your hair.
Making new molecules, and finding out what atoms make up old ones, is called chemistry.