In the bar on top of the post window, there's a little plus symbol. In there, there's a spoiler option. This will place two BB tags that look like this: [spoiler][/spoiler] Put whatever you want between these tags, you can also just write them out. Alternatively, write what you want to spoiler first, then select it with click&drag, then press the spoiler button. It will place the tags around the text you've selected.
On a related note, in order to make the system ignore formating and thus show tags like that, write [plain][/plain] around whatever you want to show someone.
