Ionic bond vs. Covalent bond has to do with electronegativity. Both kinds of bonds involve the valence electrons of the atoms involved, the question is, which one wants those electrons more? The greater the difference between the electronegativities of the atoms, the more likely it is that the bond will be Ionic rather than Covalent.
The best way to compare and decide if a bond is Ionic or Covalent (for me anyway) is to compare the positions of the bonded atoms on the periodic table. The further apart two atoms are on the periodic table, the more likely they are to be ionic. So looking at your examples, KBr is ionic, as potassium and bromide are on opposite sides of the periodic table. Bromine needs an electron to get to noble gas configuration and is very electronegative, potassium has an extra above noble gas configuration and thus is not very electronegative at all. Bromine will be able to strip that electron from potassium.
Hydrogen Sulfide actually kind of breaks this rule, it's compositional atoms are on opposite sides of the periodic table, but it is considered covalent rather than ionic--similar to water. I don't know how much you've learned about acids? But Hydrogen Sulfide is a weak acid. It does not dissociate entirely in water, while the strong acids do, and most are considered ionic.
NF3 contains two atoms from the same side of the periodic table. It's going to be covalent, since the difference in electronegativities will be rather negligible.
As Veks said, Carbon is one thing you'll essentially never see as an ion. It will exist as an ion for brief periods ONLY as a reaction intermediate, and you'll be seeing that later in the year. But ionic bonds will dissociate from each other in solution, and carbon just won't do that. Not when it would be left with nothing but hydrogens to balance its charge, anyway. Methyl chloride is covalent.
To be honest, though ... that's a pretty mean question. There's not really any set easy formula for figuring out ionic vs. covalent ... it's sort of a gut instinct based on experience with chemistry.
Basically, if you know it'll dissolve in water, conduct electricity, and is present in a solid form ... it's probably an ionic compound.