Exclude category

Ok, I did this before but removed the code and didnt save it.

But I would like to exclude a category from my front page.

If I recall, I just added a one liner to my index.php , I believe it was something like … exclude=18
But I don’t remember what it is.

I also dont remember if removing it from the front page removed it frmo the RSS feed .. if not how would i do that?

I know there are plugins for this but I would like to use code.