That depends on the code and on how "deep" you need to know it... I propose the following order:

1. read the documentation (user manual, requirements, architecture, design)
2. try to understand the related module-testcases
3. read the comments inside the code
4. read the code itself
5. execute the code with debug-output, possibly alter the code and determine why which module-testcase fails

If the documentation is sparse, you might use re-engineering-tools like doxygen, or this resource for regular expressions.

Good luck (especially if you don't have documentation nor comments ;-)

Rata