Hi Mindsigh! Use New Topics for new letters that you create/originate. Use Post Reply when responding to letter posts of other people. You did it right, as it appeared as a new post.
I first heard about Classic Autism in the late 1970s or early 80s, but it didn't bear any relation to me. I first learned about Asperger's about 8-10 years ago and immediately saw myself in the article I read. I continued to read any other such articles I came across over the next few years and saw myself in each one. I finally started researching it online, and took an online test, which confirmed my belief that I have Asperger's Syndrome. It was a great relief to finally know why I am so different from most people. I eventually found Wrong Planet and lurked here for about a year before joining. The site is interesting and has been helpful. I hope you like it here, too. 
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If a man does not keep pace with his companions, perhaps it is because he hears a different drummer.
Let him step to the music which he hears, however measured, or far away.--Henry David Thoreau