New Dog, Very Challenging
A lot of what you need is in post 8. Stop putting your dog and other peoples dogs at risk! Do read the general "warning" on "Why Dog Parks are a Bad Idea." Save general warning for "meeting other dogs" I just say .."NO."
My policy is not to allow my dogs to interact with "dogs that I do not have control of or know personally are safe!" My dogs don't have problems created by "bad encounters with unknown dogs."
I believe you can fix this...link there on Dog reactive dogs but not if your going to do dog parks or "I thought my dog was friendly folks??"
If you continue to do that then, this will most likely come in handy:
Leerburg | How to Break Up a Dog Fight Without Getting Hurt!