fontlab. you have to buy it or acquire by other means. there's some tutorials on how to use it on youtube. it's got a little bit of a learning curve but it's not too hard
Like I said I do it casually and sometimes for design projects. I haven't yet made a full ascii font set.
But if that is your goal. You need to finish designing all alphabets and symbols and finish digitizing it. If you want open source, Inkscape or Gimp will do just fine. Then you import these characters to Fontlab Studio and that will help you create your own font.
Before you jump into all this, spend a lot of time at /r/typography and ask a lot of questions in the comments. Know the basics and the function of important terminology like ascender, serif, cap height, stress, baseline, stroke, bowl, spine etc.
Typography is more than just about making fonts. It is minimalist design at its core. If you go search for free fonts, you'll be bombarded with trash and overly stylized fonts - This is what you want to avoid.
It's not easy, but if you are determined, its not difficult either. Good luck!
Edit: I recommend you watch the Helvectica and Linotype documentaries. They are pretty much a required viewing for anyone remotely interested in type.