Sought help with form "How can I show / hide a textarea when

Hello,

I'm trying to do with "How can I show / hide a textarea when a checkbox is clicked?" Form

I can not get it together so as to form and get it working, who has experience and can and want any help.

You can also make me an example so I can see what I'm doing wrong. :-(😢

Greetings Jan Wink

p.s

Sorry for my poor english. I use a translation program