Where do I start?

I've started playing HandOfBrand's Lua tutorial, and I'm on the 'Intermediate' part. Is this a good place to start, or should I switch to something else?

I especially like GUI games, which might decide what I make in the future.

Intermediate is probably not the best place to start, unless you have a fair amount of basic scripting knowledge. I suggest you go to the ROBLOX Wiki. The wiki is a great source for scripting, there are tutorials, as well as detailed information on all objects, methods, functions, events, etc. that pertain to scripting in ROBLOX. It can be found at Wiki.ROBLOX.com. Here you can find scripting tutorials. http://wiki.roblox.com/index.php?title=Lua_Help

Also, this might be a good place to start with scripting. http://wiki.roblox.com/index.php?title=Absolute_beginner%27s_guide_to_scripting

:) Good luck!