So you want to start doing SharePoint Development? First things first you'll need a development environment...have a read of:
- Building a SharePoint 2007 Development Environment
- Building a SharePoint 2010 Development Environment
Secondly, I always recommend the first thing is to get your hands seriously dirty and run through a few labs:
OK, now that your confidence is up check out what others are doing in this space:
And as you want to specialise in particular areas I would strongly recommend buying a book that focuses on it like Workflow or BDC etc.
Now you're on your way, you need to keep your hand on the pulse by following the community...
...and also have confidence that there are plenty of people out there developing on top of SharePoint:
...but most of all...as you explore SharePoint Development, share your thougth process and learning curve with the community by updating this wiki...please! 
