If you are looking to contribute to Hurd-L4 and you don't know where to start don't expect the maintainers to have a fully delineated chart of things to do next. In the current state of the project, there are so many things that need to be done that a list really isn't necessary.
If you are a programmer with at least experience with C and you've never hacked anything close to the Hurd before but you really want to start contribute some code, below are provided some readings which will help you gain familiarity with microkernel based OS design and programming for Hurd-L4.
Some other requirements for hacking Hurd-L4 are low-level knowledge of how a computer is executing a program and computer assembly, although this is not required for much of the Hurd-L4 codebase since higher-level abstractions will be/have been created.
Currently Memory Management and the Device Driver Framework are hot topics still up for discussion in the system, if you would like to start working on any of these subsystems but don't have a clue how you should read the current plans in "The Hurd on L4", read some academic papers on implementations in other systems, and look at existing code (Linux, *BSDs). You should definitely understand virtual addresses, paging, and different pros and cons to paging schemes so you are able to solve problems within our own paging scheme.
Understanding the boot process is very essential to getting started. Read the Multiboot Standard and have a look at the sample kernel provided at the GRUB webpage. Also have a look at either Kickstart (included with L4::Pistachio) or laden (in the L4-Hurd CVS tree) to see how Pistachio is getting loaded into memory.
There is much to learn to get started with hacking Hurd-L4, but consider it an enjoyable learning process.
Links pertaining to learning L4-Hurd
Some of these links are at other web sites not maintained by the
FSF. The FSF is not responsible for the content of these other web sites.