That’s a pretty broad question you are asking and I’m not sure what kind of advice you might be looking for. I’ll take a stab at a couple areas and maybe it will help.
The algorithms for converting between various number systems like decimal, hex, binary, octal, etc, are pretty straightforward. Just about any programming language would serve that purpose. If you are already familiar with a programming language, that is certainly a good reason to go with it.
If you are asking about how to do those conversions, ehow.com has instructions for several of the conversion techniques. It doesn’t tell you how to program them, just the basic math steps to go through. It is up to you to translate those steps into the programming language of choice.
Let us know if you have any other questions. Best of luck!