While it may be tempting to think of the most audacious improvement you could make to PostGIS, and try to tackle that, working on an open source project is a collective endeavor, and so the first thing you need to build is not software, it is trust.
- Contributors with a history of constructive work with other members of the community get more of a hearing than folks with a grand plan.
- The way to build up a history is to choose very small pieces of improvement, and move them through the process of acceptance.
- Make small enhancements, to code and documentation, and get a feel for what improvements people are looking for.
- Communicate your interest in contributing, and discuss your plans on the postgis-devel list before investing a great deal of time.
Before you can contribute anything, you need to be able to build the software and documentation.