How can I cut fudge smooth and uniform?
The first question is how fussy you want to be about uniformity.  Many people are satisfied just using a long, thin knife to score the fudge in roughly straight lines.

If you want professional-level uniformity in the size of pieces, you'll need to lay out a grid with a straightedge, measuring equal distances on all four sides, and then use the straightedge across the fudge to score it.  (If you plan on doing this repeatedly, I'd recommend preparing a template on a piece of cardboard or something, so you don't need to measure equal spacing on the edges each time.)

As for smooth cutting, there are standard recommendations: 


Score the grid onto the fudge with a sharp knife when just barely set but still warm.  This will make the final cuts easier, as well as giving you guidelines to follow.
Everyone has an opinion about the best implement to do the final cuts with.  If the fudge is still slightly warm (but firm), a plastic knife may work best.  If the fudge is cooler and harder, a thin smooth metal blade is often easiest (usually with no sawing motions; just pressing down).  A long blade makes for quicker work.  Some people like to use a bench knife pushed straight down; others use a pizza wheel or smooth pastry wheel.  It all depends on your technique, as well as the consistency and temperature of the fudge. 
If you have difficulty cutting or if the fudge crumbles with the cuts, try warming the metal knife (or wheel or whatever) in hot water between cuts.  Wipe the hot knife dry before cutting.
Whatever you use, always wipe the cutting implement clean between cuts.


Some people claim to use dental floss for cutting.  While I've used that for cakes a few times, it seems overly fussy and difficult for fudge (but I've never tried it), though you might consider using it for fast scoring.