How do you write the formula for an ionic bond where one element is a transition metal?

I'm assuming you are wondering about a name like nickel (II) chloride or iron (III) oxide. The number in parantheses tells you the oxidation state of the transition metal atom, which is also equal to the number of electrons it has given up to form an ion.  So the Ni (II) is actually the Ni++ ion, and the Fe (III) is actually the Fe+++ ion.

As for covalent and ionic bonds, this is mainly memory work. You will have to learn and remember how well different atoms either give up or accept electrons to form ions and decide from that whether a bond is covalent or ionic. It is a gebneral rule that the more strongly an atom can give up or accept electrons the more ionic is the bond between them.