best way to secure PC for teen use

Good morning
 
what better way to secure the family PC for a teenager?
 
Basically, I wish my son could:  
- do flawless research on adult content
- play on geforcenow/steam
- watch YouTube without being able to comment or post videos
 
I had created a fairly small Google teen account, but it was easy for him to create a new Google account to post on YT...