How can I define intermolecular forces?
Things that hold molecules together.
Intermolecular forces are attractive forces that hold a molecule (not an atom!) together. There are many types, such as covalent bonding, metallic bonding, hydrogen bonding, van der Waals' forces, ionic bonding, etc.
Water is a primary substance that bonds with covalent bonds. Each hydrogen atom shares two electrons with an oxygen atom.