I see. Hmm... this might be not the right place to explain it... Whatever...
When you are posting you can see a text "Other Styles" on the left upper corner. Click it and choose "Spoiler". Then you can write the text you want and press "Ok". And tadaa, you have a spoiler box. The other way is to do it manually: write [spoiler*] in the start of your text and [/spoiler*] in the end(of course without those *-marks.)
EDIT: Damn! BlackDemon was faster!
EDIT2: And Chekhov was also faster! Damn! Why I'm the slowest one? Always!!!