Do a few searches, you will find TONs of topics on greenstuff.
Also known as "epoxy putty".
My personal answers to your questions are;
-Which is the best type/ brand?
Best is subjective given what exactly you want to do - some projects require putty with a longer cure (i.e. more working time) others are fine with shorter duration. Also, depending on the type of detail you want to do, some are better than others.
As a note; never buy
GW's version - its just rebranded blue/green putty at about 10x the price.
my personal favorite is aves apoxie sculpt.
its cheap, dries rock hard and sandable.
It does not have as long a working time as milliput or green/blue, but for my projects (more simple) it works great.
-Can you recommend any tutorials for its use?
None that I know of, check miniwargaming . com or youtube.
I am sure you will find tons of hits.
-What tools do I need?
get some basic mini sculpting tools, toothpicks, etc.
I use dental picks as well.
You can usually find cheap packs of tools at craft stores for cheap.
My apologies for not having better suggestions on tutorials or resources - others I am sure will have better suggestions (or do a search...).
Here are a few posts from some of the blogs I read;
http://fromthewarp.blogspot.com/2012/06/how-to-sculpt-greenstuff-cloaks-and.html
http://fromthewarp.blogspot.com/2009/07/working-with-greenstuff-how-to.html
best of luck and have fun!