How to exclude some post from the home page (recent post page)

I would like to exclude post from one of the categories. I found something like this: https://stackoverflow.com/questions/4476653/how-to-exclude-posts-from-certain-categories-in-latest-post . But I don’t know where to put that code. Some people says that I must put this via PHP Widget. But I do not have and acces to this widget because there’s no options for this in my wordpress admin page. Where to put that code? My theme is: https://dinothemes.com/2012/04/pizza-wordpress-theme/ . Please give me very detailed instructions because I’m a beginner at wordpress.