Help will be welcomed on any topic - development, testing, infrastructure, documentation, web design or anywhere else you have the time and inclination to offer it.
The Wiki has some suggestions for getting involved.
The bug tracker covers the project and also its infrastructure.
The sanewall source code repositories are linked to from here. Sanewall uses Git as its SCM system. If you want a quick way to get a hosted repository to fork the code, GitHub is a popular choice. See also this FAQ about building from the repository.