What are the so called clouds of dust and gas made of? We constantly read about "clouds of dust and gas" surrounding a (or being the initial state of a) star, but what elements constitute such dust and gas?
The clouds of gas and dust that form stars are usually what are called Molecular Clouds and Giant Molecular Clouds (GMCs).

The "Molecular" means that most of the atoms are combined into molecules rather than being free atoms.  So the hydrogen is mostly H2 and not H.  This is really important, since a cloud comprised of molecules can radiate away the heat of collapse much easier than a cloud of atoms can. If that heat isn't radiated away, the cloud won't collapse.

As far as the contents go, clouds int he solar neighborhood today have a composition very similar to that of the outer layers of the Sun: around 90% (71%) hydrogen, 9% (27%) helium .08% (1%) oxygen, .04% (0.5%) carbon and lesser amounts of things like nitrogen, silicon, magnesium, neon, iron and sulfur.  Note that these are atom% followed by (mass%).  See this for a table.

Astronomers would call it 90% (71%) hydrogen, 9% (27%) helium and less than 1% (2%) "metals".

Much earlier in the universe's history the percentage of hydrogen was higher and the percentage of everything else was lower.   Since the Sun is a relatively new star, the gas it condensed from was "polluted" by the gas emitted by generations of older stars which enriched it with helium and "heavy metals".