Flash? Javascript? C++?

1. Sure, why wouldn't it be? C++ will be of little use to you in itself when writing client-side web scripts, so knowing javascript will be very useful

2. Little note: Flash is not a language in itself, but a platform. Flash uses a scripting language called actionscript which is a dialect of ECMAScript, which makes it a relative of javascript as javascript is also an ECMAScript dialect. Which language to learn will be up to what you want to do with it. Flash is a platform mostly for multimedia content (mostly inside a web browser) while javascript is also suitable for general-purpose client side scripting for web browsers.

3. You get started with it like with any other language, look for learning materials (books, tutorials, online references, etc.). Knowledge of C++ should provide you with a good understanding of general programming, so it shouldn't be too hard for you to pick it up.

I personally used ActionScript for over 6 years, have to say it was fairly simple to start with, learn pretty much all of the common programming techniques.
Only now do I feel comfortable going into C++ which is a bit harder, but still a piece of cake to understand because of previous experiences.
I did however feel that Flash Games in general are a bit limited, which is why I'm going into C++ now, but don't get discourages. Actionscript 3.0 with all the new updates and the newly added Stage3D you have amazing performance, proper 3D graphics support and best of all you can create games for smartphones.

Ignatus Zuk

Another thing I noticed was that C++ is not web-based

It is still in its infancy, but you may want to take a look at WebGL
http://en.wikipedia.org/wiki/WebGL
It is a subset of OpenGL that you can access via JavaScript, via Web browser. If you want something more "web based", maybe take a quick look at it.