space after paragraphs

I am brand new to WordPress and actually to html and css, although I have been working with the web for quite some time. I am very impressed so far, and I look forward to converting my website to this easy to update system.
My question is:
How do you make the space after paragraphs smaller?
How do I change the code in index.php or in wp-layout.css to make the space between (or after) paragraphs smaller?
I think I understand the way that you make the space between lines smaller, but not paragraphs.
thanks,
Dr. Ernest Duke
An American professor teaching in Moscow