If you're a long-time member, the only way I know of to find your first post is through the premium access. (Other folks, if you know another way, please correct me). The premium access lets you see your earliest posts because it accesses the archives.
If your first post happened to be in a longterm thread, like the ones in ICR, you can look there too. It depends on what forum your first post was in. If it was in General, it moves so fast that you probably won't see it again unless premium. If it was in a slower-moving forum, then click page 25 or whatever of that forum then work your way forward and look for any threads that you might have written in.
Sorry I don't have a more precise answer. Good luck!