Use css in postings?

Hi Guys,

I want to use a .css file inside a post on my blog. Is this possilbe?
I tried to make a new post, i used the html editor in wordpress and tried to use a class of the existing .css file of my template. I tried to use a div tag to realize it. But when my post is released the div tag is missing.

So my question to all of you:

How can I use a .css file in a wordpress post?

Thanks for your help!

regards.