SECOND TIME. LAST TIME, FOR DESCRIPTIVE.
That isn't really helpful...
The simplest way of making a spoiler is hitting this button (As omega tried to explain):
Entering the title/description in the box that follows and adding content.
Or the harder way, for making spoilers you simply use the following tags...
1. At the beginning of a sentence/paragraph you have the opening tag, in this case for spoilers (Without spaces): [ SPOILER="blabla" ]
( Where "blabla" is the title/description of the spoiler)
2. Then you have the context, what you want in the spoiler: blabla
3. The closing tag: [/SPOILER]
name