Since you're probably just going to do a logo or two, I agree with mtredmon that you should just download one of the free programs. The adobe products (illustrator, photosphop, etc) are pretty costly.
Generally, for logos and the like, you want to use a vector-type graphic instead of rastor graphics. The reason being, you want it to be capable of being changed to any size without losing resolution. I have the Adobe CS4 suite which includes Illustrator which I use for vector graphics, but the "inkscape" link provided by mtredmon is probably a good alternative. However, something as small as a maker's mark, you could probably get by with using a rastor graphic (photoshop, the gimp which is free) because you won't ever need to size it extremely large.
Edit* and I say all this knowing almost nothing about knife-making or stencils. I am about a month and a half away from a degree in graphic design though.