is an open source platform for mathematics and computation that ties together many C and C++ libraries with Python code. You could browse the project bug and enhancement tracker
or the sage-devel google group
for some ideas of where to contribute and the project culture. There are a very wide range of things to do.
With Sage, and probably most other projects, a very good way to get started is to help improve the documentation. This is not glamorous, but it is effective. It involves you with other people in the project, and gets you familiar with their development practices. Its usually well appreciated since documentation quickly gets out of date on an active project. Writing documentation will suggest some coding projects naturally - to adequately describe a current bug to users, for example, you will have to understand it somewhat, and that might suggest a solution.