Documentation
There is extensive AutoIt documentation available and this documentation is your recommended reference. Many developers keep the documentation or AutoIt Help File, as it is better known, constantly open to look up functions and keywords.
Using the help file
If you have AutoIt installed then you can find it in the SciTE editor if you press [F1], or in the start menu under AutoIt > AutoIt Help File.
There are three common methods of using the help file. You can use the Contents tab to browse the help file by category, which allows you to quickly find the resource you're after based on the type of script you're writing. Alternatively, you can use the Index tab to search for a resource page by title; this is practical if you know the name of the function you're after. Finally, by using the Search tab you can search all documents for a word or phrase. This is helpful when looking for words that may appear in multiple resource pages.
As another alternative, if you are not finding the resource you're after, you can use Google to search in the online documentation.
When in doubt, if you cannot find the answer to your problem, use the forums.
Offline
An offline copy of the help file is available for download on the official AutoIt home page. You download the AutoIt Full Installation or the AutoIt Self Extracting Archive which both contain the AutoIt help file.
Source
The source code for the help file is available to anyone so that they can expand, modify or translate the original. After editing, the help file source code can be submitted to the development team.
Online
There is also online documentation available if you do not have or want to download the offline version. This is less convenient to use for regular development since some features that the offline help file offers are not supported.
The online copy can be found here: Online documentation
If you'd like to try AutoIt Online in your Webbrowser you can use script-example.com free AutoIt Web Compiler
Searching
Similar to the forums, you can use Google or your search engine of choice as a valuable searching tool. You start your query with site:http://www.autoitscript.com/autoit3/docs/ and then your search query.
A finished query may look like:
site:http://www.autoitscript.com/autoit3/docs/ GUICtrlCreateButton
Other languages
The AutoIt help file is available in a few other languages beside English.
You can find these help files here: Non-english documentation