***** NOT A TEACHER *****
I am 99.99% confident in telling you that the verb "post" in those sentences should be in the past:
"This is my first post, and I am not sure if I posted it in the right forum. Please forgive me if I posted it in the wrong forum."
I also believe that the present perfect would work there: "I am not sure if I have posted it in the right forum."
May I respectfully make a suggestion? I have found that if we members ask one question in a thread, we will get faster and more responses.
Student or Learner