ergophobe - 3:00 am on Jan 31, 2012 (gmt 0)
I'm not sure what you're thinking about when you're asking about where to begin, but I have two answers.
1. Technical - throw the project up on Github. It's free for open source projects and has all the tools you'll need. If you're going open source, you'll need to get it into some public version control repository and Github seems to have the momentum right now.
2. Marketing. This is the hard part. You need to sell people *why* they would a) use your package for a project when other options have longer track records, more active communities, etc, etc. and b) why they would contribute to your package when other packages are more mature, etc, etc.
#2 is the big challenge. I built a few custom CMS before committing to Drupal and Wordpress and I find I can just get a lot more done with those than starting from scratch. I always thought that I would come up with something "just right" and then be able to reuse the code, but I found ever site has so many special cases.