So, you're stuck? Me too, it happens a lot more than I'd like to admit.
Other than searching the web in general, luckily there are a bunch of places to get help for Unreal Engine.
Search the engine source
If you're not sure how to do something, whether a function exists for it, or how to use a function you've found, I often find the quickest way is to search the engine source code. This is my first port of call, and I find it easiest as I'm usually in Visual Studio anyway.
In Visual Studio, press Control + Shift + F to do a plain-text search, make sure "Entire Solution" is selected so it seaches both your project and the Unreal Engine project.
Unreal Slackers Discord
The Unreal Slackers Discord server is an incredibly helpful and friendly place. If you have C++ or UMG questions in particular, you will probably get a reply.
Of course it's only people helping out each other in their free time, so don't be too demanding :)
Engine Feature Samples
In the Epic Games Launcher, under Unreal Engine there is a Learn tab that has a fantasty selection of example projects and demos to learn from.
If you are new to Unreal Engine, I recommend starting with Content Examples. For a more complete example to learn from, scroll down further and try the Action RPG game sample.
The Unreal Engine AnswerHub is a Stack Overflow-style question and answer forum that anyone can use.
Unreal Engine Forums
I haven't used the Unreal Engine Forums for asking questions very often, but they are a great repository of knowledge.
Searching "Unreal Engine" on GitHub might not be a great way to answer questions, but I've found it really useful for finding interesting code snippets, new ways of doing things, plugins and more.
Definitely worth checking out every month or so to see what pops up.
Unreal Marketplace Search
Similarly, the Unreal Marketplace has a huge number of free code plugins that can be interesting to learn from.
Unreal Developer Network
UDN is available to developers that apply through Epic's developer program. It is a closed developer-only question and answer forum, similar to Answers, but 99% of the time your questions will get an answer from an Epic staff member within a week.
It costs money to be registered and access but I've found it really useful asking about odd bugs or asking if there are better ways to solve certain problems. If you're a medium to large studio I would definitely recommend it.
Unreal Engine Documentation
There's a reason the official documentation is this far down the list. It can be useful to get a high-level overview of a system but if you're writing C++ pretty soon it's better to start looking at the engine source or examples of usage.
Unreal Engine Community Wiki
The old Unreal Engine Community Wiki was a huge repository of knowledge created and maintained by hundreds of developers. Unfortunately it was removed and all the links to its pages are broken and now point to this forum post.
Michael J Cole created a backup of the original Unreal Community Wiki which is also pretty helpful.
- Tom Looman has some very interesting tutorials.
- Unreal Tournament GitHub repo
- Helium Rain's source code is available on its GitHub repo, and uses mostly Slate. It's a great place to look for examples of Slate code.
I've found Twitter a great place to learn little bits and pieces for Unreal. Here are some people who post interesting stuff!