you use BBCode tags, in this case, either of these will work (don`t mix them):
either [ spoiler=tittle(optional) ] anything you want spoilered [ /spoiler ]
or, replace spoiler with collapse.
Just remove the spaces around the brackets, now, PLEASE, be a good poster and always close your spoiler tags. The tag CAN work without the ending, but the time someone will quote you, his quote will be spoilered. So, even if your entire message is spoilered, please, close your tags.
The tittle is opitional, you could omit the =tittle.