A mineral is a naturally occurring inorganic solid, with a definite chemical composition, and an ordered atomic arrangement. This may seem a bit of a mouthful, but if you break it down it becomes simpler.
Minerals are naturally occurring
They are not made by humans
Minerals are inorganic
They have never been alive and are not made up from plants or animals
Minerals are solids
They are not liquids (like water), or gases (like the air around you)
Minerals have a definite chemical composition
Each one is made of a particular mix of chemical elements
Minerals have an ordered atomic arrangement
The chemical elements that make up each mineral are arranged in a particular way - this is why minerals 'grow' as crystals