If you want to write spoilers and keep those hidden from new visitors, here's how you do it!
Visual Editor[]
Typing {{ will bring up the "Add a template" window. Type and select "Spoiler" and click on "Add template". Then click on "Text" and write the spoiler down in the field.
Note: This textbox uses Source Editor markup.
Source Editor[]
Using the Source Editor, you can type the following:
{{Spoiler|Text=}}
Then simply put the spoiler text behing the =
Note: This method uses Source Editor markup as well.