If you are replying in an existing thread, click the REPLY button. When you do that, there are a whole bunch of formatting buttons. One of the buttons towards the end is an "S" with a "/" through it.

That is the SPOILER button. When you click it, it will put 2 tags in your text.
Anything in the middle of those 2 tags will be hidden.
If you have already typed something and later decide to hide it in spoiler tags, just hilight the whole text and then press the button. It will put the tags at the beginning and the end