what is the difference between ionic and molecular/covalent compounds? Which one uses prefixes? Which one has a charge, shares electrons, use Roman numerals, are between a metal and nonmetal, etc.? i'm really confused. please help! thank you!

Some of your questions are confusing to me, too.

Generally, ionic compounds are formed by elements in which the metal donates an electron (or electrons) to a non-metal. The metal then has a positive charge (it's called a cation) and the non-metal has a negative charge (it's called an anion).
Covalent bonds are those in which the elements share (the electrons are not transferred) the electrons. Roman numerals are used in naming compounds and it doesn't matter what the bonding is. Prefixes are part of the nomenclature and they don't depend upon the bond type.
Generally, ionic compounds are between metals and non-metals (groups I, II, III) and groups (V, VI, VII); i.e., between elements on the left side of the periodic table and the right side of the periodic table. Covalent compounds are formed between elements on the same side of the table OR between the middle of the table and either side. For example,
ionic: Na and Cl, Ba and O, Al and F. etc.
covalent: Cl2, H2, N2, HCl, NO2, CH4