You should learn a web framework like Django or Lavarel or whatever first, and then you can write an ad server, gif server, whatever.
If you are Google-sized, prefer C++ or Java. Otherwise, whatever you already know :)
I’m fairly proficient with Django. But I just can’t imagine an ad server running on Django other than as a proof of concept?
Why learning a framework would be the first step in building an ad server? I would actually go against frameworks when building ad server and focus on a custom solution from the ground up.