Personally, I started by just grabbing the source code or man pages and digging through it with a goal in mind. It really depends on what you're trying to do. Some understanding of basic tools (grep, less, find, man, C language, Google search) is really all you need to get started, then it's just looking things up, making deductions, etc...
Basically it's all trial and error. Keep notes to track your thought process. Don't be afraid to be wrong. Don't be afraid to ask other people questions, too! My number one rule is "never be afraid to look like an idiot"
