I just thought i would post some tips/links about how to get to grips with drupal which i hope you will find helpful:
When you apply it is generally a good idea to start looking into how Drupal is being developed, to get familiar with the initial tools, and ways. It is a good knowledge anyway, using CVS, learning how patches work, the ability to adapt to a coding styles is important in other projects too.
CVS
Drupal uses the CVS as version controlling tool, you can read about it, and get hold of the latest sources in the main CVS repository: http://drupal.org/node/320
Diffs and Patches
You can read on how to create patches, diffs here: http://drupal.org/diffandpatch, but there is also a nice tutorial video from add1sun on the Dojo group's page: http://groups.drupal.org/node/3181
Coding style
Developers use one general coding standard (http://drupal.org/node/318). In the beginning you probably want to look for some small, but still open issues in contributed projects, on which you can practice Drupal coding and adapt your coding style. Maintainers are often keen on the coding style, if you are not following them, your patch may very well be just rejected.
Some helpful notes can be read on the ViM editor here: http://drupal.org/node/29325 (configuring your .vimrc).
Modules
how to go about developing modules (do have a look at the example modules, mess around with code, break an fix...do the usual :P):
http://drupal.org/node/508
Again it is a good idea to check out previous Dojo lessons, the first lesson for example teaches you how a module can be built from scratch.
Drupal API
In a module there are functions. You probably dont know them yet, but fear not! The great help of developers, the Drupal API is out there, giving aid for those who need it. All you need to know about the drupal hooks can be found here: http://api.drupal.org/
and finally the mailing list: http://lists.drupal.org/listinfo/drupal-devel
Drupal Dojo
You can find many valuable lessons, notes and other material on the pages of the Dojo. For starter you can check out the lessons, lesson notes here: http://groups.drupal.org/taxonomy/term/1245.
I would advise that you at least have a bit of look around/read so that if/when you get accepted you have a good idea how things work thus only need to concentrate on the best way to go about carrying out your project.
Note by add1sun - This is a great starter list. The Drupal Dojo has started up a work area to create a SoC handbook at http://docs.drupaldojo.org (need to login to edit with a drupal username) and any suggestions or tips that can be added there would be awesome. It still really needs plenty of fleshing out. I've already copied over some of the links that shakur had here. Also NOTE that this is a temp workspace and our final product will be moved to the d.o handbooks, so don't worry about splintering and such. Thanks. :)