I am the webmaster for the site John and I run and I can say our experience has been positive with Yahoo. I can upload all sorts of stuff and never worry about running out of space( I've got a forum living on there now, ready for when we go live with Airwolf.)
Using free webspace is NEVER a good idea if you want to do anything remotely complicated. Selling stuff and forums won't be possible with free web hosts, unless they're a lot more generous than when I remember them.
As for security, it doesn't matter what you use to code it - security is up to the web host, really. I code manually, but some people use expensive web editors like Dreamweaver. Either way, the code that comes out the end is usually the same - HTML(You could look at the source of our website for a start...)
You'll want to lean at least the basics of HTML, even if you use a drag-and-drop editor. There's some stuff available for linux that will do very well to code in, too. Look in your package manager.
There are loads of forum scripts about, but the one I am useing is called SMF. It is really quite good, but VBullitin, yaBB and so on will work as well. Just make sure it will work on your server. Some use a thing called PHP/MySQL, some will use ASP or Pearl. These are different scripting languages, a bit like addons for HTML. PHP is probably the most common.
As for open source, it doesn't matter in this case. As far as I know, there is no clause anyware in the ELUA of Frontpage or any other web editor that I have that restricts what you can do with what you make. You can use Notepad or Kedit, Dreamweaver or QT, it won't matter.
On the other hand, the web host might. Generally though, restrictions only apply to free or really cheap webspace.
Sorry for going on a bit, if there's anything you don't understand, don't hesitate to ask!
I fly through the valley of death, but I fear no evil,for I am at 80,000 feet and climbing. Let's powerdive and scare that poor soul in that Ultralight there.