How do I write out chemical formulas?

Carbon tetrachloride :
The prefix "tetra" means "4". The name carbon tetrachloride shows that each molecule contains one carbon atom (C) and four chlorine atoms (Cl).
Hence, the chemical formula for carbon tetrachloride is CCl₄.

Ammonium Nitride :
The chemical formula for ammonium ion is NH₄⁺ (which carries 1 positive charge) and that for nitride ion is N³⁻ (which carries 3 negative charges). To make a neutral compound, each formula unit should contain 3 NH₄⁺ ions (totally 3 positive charges) and 1 N³⁻ ion (totally 3 negative charges).
Hence, the chemical formula for ammonium nitride is (NH₄)₃N.

Copper(II) Hydroxide :
The chemical formula for copper(II) ion is Cu²⁺ (which carries 2 positive charges) and that for hydroxide ion is OH⁻ (which carries 1 negative charge). To make a neutral compound, each formula unit should contain 1 Cu²⁺ ion (totally 2 positive charges) and 2 OH⁻ ion (totally 2 negative charges).
Hence, the chemical formula for ammonium nitride is Cu(OH)₂.

Formula writing.....

Carbon tetrachloride
Greek prefixes .... the prefix tells the number of atoms. "Tetra-" means 4
CCl4

Ammonium nitride
The nitride "ion" is N^3-, and so we would have to have four ammonium ions to go along with it so that the sum of the oxidation states is zero. Nitride has an oxidation state of -3, but it is doubtful that it has an actual charge of -3. Nitrogen has a very low electron affinity, with little tendency to add even one electron to a neutral atom. Assuming it does to make N^-, and a second electron or even a third would result in a great deal of repulsion and little chance the N^3- ion would actually form. On the other hand, it is more likely that three half-filled p-orbitals will take part in bonding with more covalent character.
(NH4)3N ...... is highly unlikely, and is not found in the Handbook of Chemistry and Physics.
On the other hand, ammonium azide, NH4N3 is a viable (but unstable) compound. Ammonium azide is represented as NH4^+ and N3^- ions. It could be written as N4H4. There are also a different compound with the same formula, N4H4, called tetrazene, represented as H2N-N=N-NH2.

Copper(II) hydroxide
Many compounds in which the metal exhibits more than one oxidation state are written with the Stock system (Named for Alfred Stock) in which a Roman numeral represents the oxidation state of the metal.
Cu(OH)2 is the formula. The Roman numeral indicates an oxidation state of +2 for copper. The hydroxide has an oxidation state of -1, and so two hydroxides are needed so that the sum of the oxidation states is zero. The hydroxide is place in parentheses because it is a "unit" and together the O and H exhibits an oxidation state of -1. Students might mistakenly write CuOH2.

Here is a flowchart for naming comounds which I prepared for my students. You may find it useful.
https://c7chemistry.wikispaces.com/file/view/Nomenclature%20flowchart%20100610.pdf

Found at: https://c7chemistry.wikispaces.com/2.+Chemical+Reactions+-+Nomenclature

Ammonium nitride will be 4NH3. It is the nitrogen analog of "Hydronium Oxide": (H3O)2O