Is it possible to chose a particular site within the content network of Adwords in order to advertise?
Yes, when you start a campaign choose - Networks, devices and extensions > Let me choose >Content >Relevant pages on the placements that I manage only > save and continue on the next page you add the site(s) you want.
What don't you get? I told you exactly how to do that. Start a campaign, specify content network, choose placements.