Adding AdSense to newest entry only

Hi all. I’m a PHP idiot, so a little help goes a long way by me. I’m trying to add AdSense to my page, but I only want one ad block to appear under the newest entry on my home page, i.e. under the first/top entry and nowhere else.

The way I imagine it is (in plain English):

If this is newest entry then
show AdSense code here
else
do nothing

Can anyone translate that into functional PHP?