This article is aimed at people totally new to Moodle development and have some PHP experience, if you don't have that experience please visit PHP for novices. It is a few tips to help you get started with the mass of Moodle code.
Please feel free to add more tips here, but I think it is also good if we can keep this article quite short.
If you have not already found it, the main source of developer documentation is Developer_documentation.
For finding where to start, you need to know that when you are looking at, for example, http://moodle.org/mod/forum/discuss.php?d=82799, then the code for that is in /mod/forum/discuss.php, and you just need to follow through what it does.
It calls functions in the main Moodle libraries, and the three most important are:
As you look at the code, it is often a good idea, to insert statements like
debugging('In function require_login');
which will print the message above if it gets far enough so you know the code has not stalled before there, or
which will print out a variable, showing you what it contains.
Variable like $course are often objects with lots of fields. Seeing what they contain will help you understand what is going on. The first of these prints out whatever text you give it, and information about the sequence of function calls that the code took to get there. (It only works if you go to Administration > Server > Debugging, and turn the debug level to ALL or DEVELOPER.)