Just combine the ion formulas to get the formula for a compound with a neutral charge.
NH4 + and NO3 -
The formula is NH4NO3, because ammonium has a positive one charge and nitrate has a negative one charge, so the charges cancel out to make a neutral molecule.
NH4 + and HCO3 -
The formula is NH4HCO3, because the positive one and negative one charges cancel out.
NH4 + and SO3 2-
The formula is (NH4)2SO3, because you must have two ammonium molecules to get a net positive charge of two to cancel with sulfite's negative two charge.
NH4 + and PO4 3-
The formula is (NH4)3PO4, because you must have three ammonium molecules to cancel out phosphate's negative three charge.