Don't worry. Nobody started as a pro. :)

First, you could check out the Elgg docs at http://docs.elgg.org/wiki/Main_Page. There are also a few tutorials included there. Then you could buy Cash Costello's book on Elgg: http://www.amazon.com/Elgg-Social-Networking-Cash-Costello/dp/1849511306. The book is also available as e-book.

But it will take time to learn regardless which approach you take. Best would be to start small. I would suggest to also study the code of other plugins. While each plugin (being coded by different developers) has surely a bit of a different style of code and of course IS different because fulfilling different purposes, you will notice a common basic structure (e.g. start.php, manifest.xml, views, forms, action, widgets, plugin settings). What you need to understand is how the code of the plugins is organized in the different files and folders. You could pick a small plugin with limited features and analyze in more detail what each part of the code is supposed to do. If you begin to understand the common pattern, you are on a good way.

Later on when developing your own code (or maybe already when analyzing the code of other plugins), for more specific details about Elgg's functions you can already look them up at http://reference.elgg.org/.