If you plan to melt chocolate, you'll have less chance of scorching it if you first chop it as evenly as possible. When chopping chocolate, tiny shards inevitably get everywhere. For easy cleanup, line a rimmed baking sheet with parchment and put your cutting board on top of that. Use the paper to gather the shards.
To chop a thick slab of chocolate, set the blade of your biggest knife (preferably a chef's knife) on a corner of the slab and bear down with both hands to break off a small bit; repeat. As that corner becomes a flat edge, turn the slab and begin cutting at another corner.
For chopping large amounts of chocolate, a food processor can work. Break thin bars into pieces and pulse in the processor with the steel blade until it's evenly chopped. (Sometimes a few pieces resist chopping; break these into smaller pieces and keep pulsing.) For block chocolate, cut the block into chunks that will fit in the feed tube and use the coarse grating disk (heavy block chocolate might damage the machine if you use the steel blade).