how do u make a spoiler?

i was wondering how do u make a spoiler? i know how to add a file but how do u insert it into a spoiler?

 

YOu don't need to upload a file in a spolier it is added, make a spolier click on the spoiler button

 

Spoiler

like this?

When you're posting, you have a couple of boxes right above where you type.
There's B, i, u etc. The one on the bottom right of those boxes is named Spoiler.