Unless you saved the original e-mail with the welcome message, there's no easy way to figure out when you registered.
However, here's how to find your first post:
1) Go to the Search page (by clicking on search, an icon at the top of the forums page).
2) Enter your user name in the Search Field
3) Choose Search by Author
4) Choose "All" Days in the past and "Search All Forums"
5) Click "Search Archived Threads"
Let the search run (if you do this on a busy day, this can take a LONG time, so it's probably best to run the search late at night or on a weekend)... when you get the list of every thread you've ever posted in, look through the old ones until you find your first post (note that threads are sorted by date of most recent post... not date of YOUR post, so the "oldest" thread may not be YOUR oldest thread).
If you have any clue what your first post was, or where it was, you can narrow your seach considerably... good luck!
You never know what might be up my sleeeve...