How to use JS on my blog.

Answer 1

you can use <script> tags in html files
write js code in this file
or in other file
then use src="path of the js file" in script tags
to include it
just learn this js couses and you'll know how to do it.

Answer 2

if you don't own your blog domain then it can be disabled and therefore impossible.
But if you own the domain then you can use Javascript almost anywhere you want.
you just need you pu it in <script></script> tags
or in a separate js file.