For a beginner stand point, I'll start learning the SP basic, such as what is a list, library, tasks, web application, site, site collection, security, etc. Than as you grow as a developer you can learn all the others tricks. Don't worry, you won't and to not need to know everything to develop your first web part! I've learn a lot doing some SP projects and I'm far from mastering everything, but I get closer everyday ;)
Take a look at the Learn section on Technet, Walkthrough and evaluation guide and the SharePoint 2010: Developer and IT professional learning plan. You can also read a book (I'm sure they have some out there) or take a class(online or offline). Online search tools are also great tools!