RTFM

RTFM means Read This Fine Manual

Here is a list of fine manulals related to Dynamics Ax development:

Developer's guide -- go Help > Developer's guide at window menu of Ax or here
Best practice -- "C:\Program Files\Navision\Client\Bin\Dev_BPHB.chm"
System Documentation -- open AOT scroll to end of AOT for the System Documentation node expand and click on classes functions etc
Source code -- if you can not understand how to use function see it's cross-refrences (AddIns submenu of context menu of AOT node) to look for examples. Hint: System Documentation nodes can have cross references too. It is also useful to read source of the Global class and other system classes (at the end of the 'classes' branch in AOT). See also Tutorial_* forms and classes.
AxSearch -- search internet sites related to Dynamics Ax
http://google.com -- search other sites. By the way, you can use google to search through MSDN. For example try the following query "GroupPrompt property site:msdn2.microsoft.com"