Butcher, I'm the most wrong guy to ask a question like this of! Dan Gray has this wonderful chart for drill and tap sizes:
http://www.knivesby.com/drill-tap-chart.html. That will tell you what drill size you need for each tap size. For example, if you want to tap a 2-56 hole, you'd want to go with a #49, #50 or #51 drill size (I think most people use #51). You can also find info for clearance hole sizes, too, but not on this chart. For example, let's say you're making a folder. One side will be drilled and tapped, the other will be drilled slightly larger to allow the screw to clear the hole so it can thread into the other liner, if that makes sense. Or, if you're attaching a scale to a liner, you need to know the clearance drill size for the screw to drill the scale out that size so it passes through cleanly.
From my extensive questioning about these subjects here, I was recommended to get spiral point plug taps, I believe. Buy a few taps, get a tapper from Higgins, and you'll be ready to rock (oh, get some tapping fluid, too)! I probably wouldn't buy a whole set of taps. I'd go for a few of each of the size you will use. Most people in knifemaking use 1-72 and especially 2-56, although you'll see some 0-80 and 4-40 (I think?) every now and then, too. 2-56 is the workhorse, though.
But, I am a real amateur, fly by the seat of my pants guy when it comes to taps, drills, reamers, etc. so hopefully someone who knows what they're talking about will come to the rescue!
